Osteoporosis International

, Volume 25, Issue 6, pp 1735–1750

Effects of sequential osteoporosis treatments on trabecular bone in adult rats with low bone mass

  • S. K. Amugongo
  • W. Yao
  • J. Jia
  • Y.-A. E. Lay
  • W. Dai
  • L. Jiang
  • D. Walsh
  • C.-S. Li
  • N. K. N. Dave
  • D. Olivera
  • B. Panganiban
  • R. O. Ritchie
  • N. E. Lane
Original Article

DOI: 10.1007/s00198-014-2678-5

Cite this article as:
Amugongo, S.K., Yao, W., Jia, J. et al. Osteoporos Int (2014) 25: 1735. doi:10.1007/s00198-014-2678-5

Abstract

Summary

We used an osteopenic adult ovariectomized (OVX) rat model to evaluate various sequential treatments for osteoporosis, using FDA-approved agents with complementary tissue-level mechanisms of action. Sequential treatment for 3 months each with alendronate (Aln), followed by PTH, followed by resumption of Aln, created the highest trabecular bone mass, best microarchitecture, and highest bone strength.

Introduction

Individual agents used to treat human osteoporosis reduce fracture risk by ∼50–60 %. As agents that act with complementary mechanisms are available, sequential therapies that mix antiresorptive and anabolic agents could improve fracture risk reduction, when compared with monotherapies.

Methods

We evaluated bone mass, bone microarchitecture, and bone strength in adult OVX, osteopenic rats, during different sequences of vehicle (Veh), parathyroid hormone (PTH), Aln, or raloxifene (Ral) in three 90- day treatment periods, over 9 months. Differences among groups were evaluated. The interrelationships of bone mass and microarchitecture endpoints and their relationship to bone strength were studied.

Results

Estrogen deficiency caused bone loss. OVX rats treated with Aln monotherapy had significantly better bone mass, microarchitecture, and bone strength than untreated OVX rats. Rats treated with an Aln drug holiday had bone mass and microarchitecture similar to the Aln monotherapy group but with significantly lower bone strength. PTH-treated rats had markedly higher bone endpoints, but all were lost after PTH withdrawal without follow-up treatment. Rats treated with PTH followed by Aln had better bone endpoints than those treated with Aln monotherapy, PTH monotherapy, or an Aln holiday. Rats treated initially with Aln or Ral, then switched to PTH, also had better bone endpoints, than monotherapy treatment. Rats treated with Aln, then PTH, and returned to Aln had the highest values for all endpoints.

Conclusion

Our data indicate that antiresorptive therapy can be coupled with an anabolic agent, to produce and maintain better bone mass, microarchitecture, and strength than can be achieved with any monotherapy.

Keywords

Alendronate Mass Microarchitecture Ovariectomy Parathyroid hormone Raloxifene Strength 

Introduction

Osteoporosis is currently treated with effective therapies that act through different tissue-level mechanisms to reduce fracture risk. These include antiresorptive agents like bisphosphonates, a selective estrogen receptor modulator, and a receptor activator of NF kappa B ligand (RANKL) inhibitor, which reduce bone turnover [1], and an anabolic agent, parathyroid hormone (PTH; hPTH(1–34)) [2], which increases bone formation. Both antiresorptive and anabolic treatments, which may include both PTH(1–34) and PTH(1–84), raise bone mass, improve bone strength, and reduce fracture risk in postmenopausal women with osteoporosis [3, 4, 5, 6].

As osteoporosis is a chronic disease, long-term management is required. Though many patients respond well to treatment for a number of years, others may need additional therapy. As a result, osteoporosis medications with complementary tissue-level mechanisms of action are not infrequently prescribed in a sequential manner. As bisphosphonates are recommended as the first-line therapy for osteoporosis, many patients that may eventually receive PTH have already received bisphosphonates. Preclinical data that compare various treatment sequences could help guide future osteoporosis treatment strategies to achieve better fracture risk reduction than is possible with one agent alone.

Some clinical studies of sequential and/or combined treatment have been reported. PTH treatment has been followed by antiresorptive agents [7, 8]. Treatment with either raloxifene (Ral) or alendronate (Aln) has been followed by PTH [9, 10] or combined with PTH [10, 11]. Preclinical and clinical studies agree that sequential therapy using antiresorptive agents after PTH is necessary to maintain the bone mass that is added by PTH [7, 12, 13, 14]. However, all clinical studies of sequential osteoporosis treatments, not only are of relatively limited duration and sample size, but also lack fracture data. Most report only bone mineral density (BMD) data, which by itself is insufficient to completely predict fracture risk [15].

Studies have also been conducted using sequential therapy on small animal models of osteoporosis [14, 16, 17]. In rats that were ovariectomized (OVX) and left untreated for 11 weeks, the increased bone volume (BV) and trabecular BMD seen after 12 weeks of PTH(1–34), was lost within 12 and 24 weeks of PTH withdrawal, respectively [17]. When PTH withdrawal was followed by risedronate, both increased bone mass [17] and bone strength from PTH were maintained [18]. However, a dose of estrogen that prevented OVX-induced bone loss in adult female rats, failed to maintain BV/ total volume (TV), BMD, and bone strength, after PTH withdrawal [17].

Despite the fact that patients are now cycled through bone active medications, there are very few preclinical data addressing how these sequential osteoporosis therapies affect bone mass, microarchitecture, and strength. We propose to address with preclinical data, the possibility that properly sequencing current osteoporosis treatment agents, with their complementary mechanisms of action, can produce better fracture risk reduction than can be achieved by any single monotherapy. To do so, we evaluated bone quantity, microarchitecture, and strength in various sequences of antiresorptive and anabolic therapy that have already been or could be applied clinically. The goal of our study was to compare a set of preclinical bone endpoints from approved therapies in which fracture risk data exist for humans, to the same endpoints after sequential therapies in which human fracture risk has not yet been measured.

Materials and methods

Animals and experimental procedures

Six-month-old female OVX or sham-operated Sprague Dawley rats were purchased from and operated on at Harlan Laboratories (Livermore, CA, USA). The rats were shipped to our laboratory 2 weeks after surgery. They were then maintained on commercial rodent chow (Rodent Diet, cat. no. 2918, Teklad; Madison, WI, USA) in a 21 ºC temperature room, with a 12-h light/dark cycle. Within a week of arrival in our laboratory, pair feeding of OVX to sham rats was started. Sham and OVX groups were necropsied at 2 months postsurgery (period 0). All remaining OVX rats were then randomized by body weight into ten groups (Table 1), which represent currently used and potential sequences of anti-osteoporosis medications. OVX rats were treated for 3 months (period 1) with vehicle (Veh; normal saline, 1 mL kg−1 dose−1, three times per week by subcutaneous (SC) injection; Life Technologies, cat. no. 10010, Grand Island, NY, USA), PTH (hPTH(1–34) (human) acetate (25 μg kg−1 dose−1 SC, 5×/week; Bachem Biosciences Inc, cat. no. H-4835, King of Prussia, PA, USA), Aln (25 μg kg−1 dose−1 SC, 2×/week; Sigma, cat. no. A-4978, St. Louis, MO, USA), or Ral (5 mg kg−1 dose−1, 3×/week by oral gavage; Sigma, cat. no. R-1402) (Table 1). No group was orally dosed three times weekly with Veh to control strictly for oral gavage of Ral rats.
Table 1

Experimental groups

Treatment group

Period 0 (days −60 to 0)

Period 1 (days 0–90)

Period 2 (days 91–180)

Period 3 (days 181–270)

Sham

No treatment (n = 12)

No treatment (n = 12)

No treatment (n = 12)

No treatment (n = 7)

OVX

 Veh-Veh-Veh

No treatment (n = 10)

Vehicle (n = 10)

Vehicle (n = 10)

Vehicle (n = 10)

 Aln-Aln-Aln

Alendronate (n = 12)

Alendronate (n = 12)

Alendronate (n = 12)

 Ral-Ral-Ral

Raloxifene (n = 11)

Raloxifene (n = 11)

Raloxifene (n = 12)

 Aln-Veh-Aln

Alendronate (n = 12)

Vehicle (n = 12)

Alendronate (n = 15)

 PTH-Veh-Veh

hPTH(1–34) (n = 12)

Vehicle (n = 11)

Vehicle (n = 12)

 PTH-Aln-Veh

hPTH(1–34) (n = 12)

Alendronate (n = 12)

Vehicle (n = 11)

 PTH-Ral-Ral

hPTH(1–34) (n = 6)

Raloxifene (n = 12)

Raloxifene (n = 12)

 Aln-PTH-Veh

Alendronate (n = 12)

hPTH(1–34) (n = 12)

Vehicle (n = 12)

 Aln-PTH-Aln

Alendronate (n = 11)

hPTH(1–34) (n = 12)

Alendronate (n = 10)

 Ral-PTH-Ral

hPTH(1–34) (n = 11)

Raloxifene (n = 11)

Day −60 was the day of ovariectomy (OVX). Day 0 was the first day of dosing. Period 0 (days −60 to 0) allowed establishment of mild-moderate estrogen-deficiency osteopenia. OVX rats were still losing trabecular bone during periods 1 and 2. The number of rats killed after each period from each group is shown. As Ral treatment during period 1 was common to Ral-Ral-Ral and Ral-PTH-Ral groups, no rats from the Ral-PTH-Ral group were killed at the end of period 1. The treatment regimens were: Vehicle (Veh) subcutaneously (SC) at 1 mL kg−1 dose−1, 3×/week; parathyroid hormone (hPTH(1–34)) (PTH) SC at 25 μg kg−1 dose−1, 5×/week; alendronate (Aln) given SC at 25 μg kg−1 dose−1, 2×/week; and raloxifene (Ral) by oral gavage at 5 mg kg−1 dose−1, 3×/week

The approved dose of PTH(1–34) for the treatment of human osteoporosis that is safe, well-tolerated, and efficacious, is 20 μg daily, equating to 2.3 μg kg−1 week−1 in a 60-kg woman. A PTH(1–34) dose that is often used in rat studies is 80 μg/kg daily or 560 μg kg−1 week−1 [19]. The PTH(1–34) dose of 25 μg kg−1 day−1 at 5 days/week utilized in our rat study is 125 μg kg−1 week−1. As we administered PTH(1–34) for 90 days, we chose a dose that was only ∼50-fold, rather than ∼240-fold above that used in osteoporotic humans.

The approved dose for Ral for the treatment of human osteoporosis is 60 mg daily by mouth or 7 mg kg−1 week−1 in a 60-kg woman. The Ral dose (5 mg/kg at 3 days/week) in our rat study is 2.1 mg kg−1 week−1 orally. The minimum Ral dose that prevents most OVX-induced bone loss in rats is 1.5 mg kg−1 day−1 or 10.5 mg kg−1 week−1 orally [20].

The approved dose of Aln for the treatment of human osteoporosis is 70 mg weekly by mouth or 1.17 mg kg−1 week−1 in a 60-kg woman. Assuming a 0.7 % bioavailability, this equates to 8.2 μg kg−1 week−1 that is absorbed. Our dose of Aln (50 μg kg−1 week−1 SC) is also based on the minimum dose that has been reported to completely prevent OVX-induced bone loss in rats [21].

After 90 days (period 1), 6–12 animals were randomly selected from each group and necropsied, whereas the remaining animals were switched to their second treatment regimen. At 180 days, another 10–12 animals from each group were necropsied (period 2), whereas the remaining animals were switched to their third treatment for 90 days (period 3), at the end of which time they were necropsied. Experimental groups, with final numbers, are listed (Table 1). During the study, nine rats, randomly disbursed over the ten OVX groups, died, leaving 383 that reached necropsy. The study protocol was approved by the University of California Davis Institutional Animal Care and Use Committee.

At necropsy, the rats were euthanized by CO2 inhalation. Whole blood was drawn by venipuncture of the left ventricle, then was transferred to a 10-cm3 centrifuge tube and spun to yield serum that was then frozen at −20 °C. The uterus was inspected visually to confirm efficacy of OVX. The right femur and the lumbar vertebral (LV) spine 2–6 were excised and cleaned. The fifth lumbar vertebral body (LV5) and sixth lumbar vertebral body (LV6) were dissected free from the vertebral segment. The right femur and LV5 were placed in 10 % formalin for 24 h, and then transferred to 70 % ethanol. LV6 was wrapped in saline-soaked gauze and frozen at −20 °C. The right femur and LV5 were stored in 70 % ethanol, whereas LV6 remained at −20 °C until analysis.

MicroCT measurements of BV, microarchitecture, and degree of mineralization of bone tissue

The right distal femoral metaphysis (DFM) and LV5 were scanned with μCT (VivaCT 40, Scanco Medical AG, Bassersdorf, Switzerland) at 70 kev and 85 μA with a voxel size of 10.5 μm. Scanning of the DFM was initiated at the level of the growth cartilage-metaphyseal junction and extended proximally for 250 slices. Evaluations were performed on 150 slices beginning at 0.2 mm proximal to the most proximal point along the boundary of the growth cartilage with the metaphysis. The entire LV5 was scanned. All trabecular bone in the marrow cavity was evaluated. For each slice, the volume of interest was defined as ∼0.25 mm internal to the boundary of the marrow cavity with the cortex. The methods for calculating BV, TV, trabecular thickness (Tb.Th), trabecular number (Tb.N), Structure Model Index (SMI), degree of anisotropy (DA), and degree of bone mineralization (DBM) have been described [22].

Biomechanical testing

Mechanical properties were determined by a compression test of the LV6. The posterior elements and transverse processes were removed with a bone cutter (Liston Gross Anatomy Bone Cutter, Fine Science Tools, Inc, cat. no. 16104, Foster City, CA, USA). The endplates were removed using a wafer saw and polished to flat, parallel surfaces. Before testing, seven measurements were taken using a digital caliper (Fowler® 0–12 in./300 mm IP54 Digital Caliper, Global Equipment Company, Inc, cat. no. T9FB799807, Buford GA, USA), that is, one length measurement (cranial-caudal direction) and six diameter measurements (major and minor axis at ∼0.5 mm from the cranial end and the middle and at ∼0.5 mm from the caudal end) of the vertebral body. The average diameter measurements were used with a standard area of a circle equation (π*((d/2)^2)), to calculate cross-sectional area of the test sample.

Test samples were stored in Hanks’ balanced salt solution (HBSS) 12 h prior to testing. They were loaded using an electro servo-hydraulic materials testing system (MTS Model 831, Eden Prairie, MN, USA) in HBSS at 37 °C at a displacement rate of 0.01 mm/s. Maximum load, maximum stress, yield stress, stiffness, and energy absorption were calculated from the load-displacement curve. Energy absorption, or work to fracture, was specifically determined from the area under the load-displacement curve divided by twice the cross-sectional area of the test specimen.

Statistics

The group means and standard deviations (SDs) were calculated for all variables (Tables 2, 3, 4, and 5). In addition, when groups were treated identically during the same period, their data were combined and means and SDs for the combined data were calculated for all variables (Table 6). The differences for each variable between the two period 0 groups were analyzed by a two-sided t test. Differences among the eleven groups for each variable within periods 1, 2, and 3, respectively, were analyzed by analysis of variance (ANOVA). Differences were considered statistically significant at p value of ≤0.05 and intergroup differences were determined by an F test with the Bonferroni correction, as a post-hoc test for multiple (pair-wise) comparisons.
Table 2

Trabecular bone mass, microarchitecture, and bone strength at end of period 0

 

Sham

OVX

MicroCT

 Number of rats

11

10

  LV-BV/TV (%)

40.0 ± 4.0*

27.8 ± 4.7

  LV-Tb.N (mm−1)

4.00 ± 0.36*

3.36 ± 0.31

  LV-Tb.Th (μm)

90.4 ± 4.8*

81.0 ± 6.4

  LV_SMI

−1.42 ± 0.5*

−0.12 ± 0.50

  LV_DA

1.73 ± 0.06

1.77 ± 0.05

  LV-DBM (mgHA/cm3)

907 ± 23*

871 ± 15

 Number of rats

11

8

  DF-BV/TV (%)

31.3 ± 4.0*

15.0 ± 3.7

  DF-Tb.N (mm−1)

4.84 ± 0.49*

2.99 ± 0.13

  DF-Tb.Th (μm)

82.3 ± 6.2*

72.4 ± 6.7

  DF_SMI

0.65 ± 0.38*

1.88 ± 0.20

  DF_DA

1.42 ± 0.05

1.46 ± 0.08

  DF-DBM (mgHA/cm3)

970 ± 14*

944 ± 12

LV compression

 Number of rats

12

10

  Max load (N)

210 ± 46

180 ± 51

  Max stress (MPa)

21.0 ± 4.2

17.8 ± 4.0

  Yield stress (MPa)

11.8 ± 3.6

12.0 ± 4.6

  Stiffness (GPa)

0.55 ± 0.23

0.40 ± 0.16

  Energy absorption (kJ/m2)

5.1 ± 2.5

3.9 ± 1.9

Mean ± SD

LV lumbar vertebral body, DF distal femur, BV/TV bone volume/tissue volume, Tb.Th trabecular thickness, Tb.N trabecular number, SMI Structure Model Index, DA degree of anisotropy, HA hydroxyapatite, DBM degree of bone mineralization, Max load maximum load, Max Stress maximum stress

*p < .05, difference from OVX

Table 3

Trabecular bone mass, microarchitecture, and bone strength at end of period 1 (day 90)

 

Sham

OVX

Aln-Aln-Aln

Ral-Ral-Ral

Aln-Veh-Aln

PTH-Veh-Veh

PTH-Aln-Veh

PTH-Ral-Ral

Aln-PTH-Veh

Aln-PTH-Aln

Ral-PTH-Ral

Group ID

s

*

a

b

c

d

e

f

g

h

i

MicroCT

 Number of rats

12

10

12

11

12

12

12

6

12

12

 

  LV-BV/TV (%)

40.9 ± 6.1*

28.7 ± 5.2

32.2 ± 3.0*bdefh

27.7 ± 2.4cdefgh

33.4 ± 3.5*defh

53.4 ± 6.5*g

51.5 ± 7.5*g

48.1 ± 6.1*g

36.1 ± 5.5*h

51.0 ± 4.4*

 

  LV-Tb.N (mm−1)

4.09 ± 0.54*

3.21 ± 0.27

3.43 ± 0.23*b

3.19 ± 0.16cdefgh

3.54 ± 0.25*

3.55 ± 0.29*

3.50 ± 0.36*

3.55 ± 0.31*

3.42 ± 0.19*

3.54 ± 0.30*

 

  LV-Tb.Th (μm)

92.0 ± 6.5

84.1 ± 6.6

88.0 ± 5.5*bdefh

81.7 ± 4.6cdefgh

89.4 ± 3.6*defh

139.5 ± 15.5*g

134.0 ± 11.4*g

121.7 ± 10.2*g

97.5 ± 12.5*h

132.4 ± 6.4*

 

  LV-SMI

−1.71 ± 0.78*

−0.35 ± 0.55

−0.52 ± 0.36*bdefh

−0.29 ± 0.28cdefgh

−0.48 ± 0.46*defh

−2.29 ± 0.69*g

−2.84 ± 1.14*g

−2.12 ± 0.78*g

−0.90 ± 0.58*h

−2.43 ± 0.60*

 

  LV-DA

0.90 ± 0.81*

1.76 ± 0.07

1.75 ± 0.07

1.71 ± 0.04

1.76 ± 0.04

1.67 ± 0.11

1.69 ± 0.08

1.67 ± 0.04

1.73 ± 0.09

1.67 ± 0.08

 

  LV-DBM (mgHA/cm3)

917 ± 16*

887 ± 13

887 ± 15defh

904 ± 17defh

901 ± 18defh

936 ± 20*egh

925 ± 26*fgh

924 ± 23*gh

908 ± 31h

957 ± 19*

 

 Number of rats

12

10

12

11

12

12

12

5

11

10

 

  DF-BV/TV (%)

29.4 ± 7.0*

12.8 ± 4.4

16.9 ± 4.0bcdefh

8.5 ± 2.8cdefgh

23.9 ± 6.0*defh

39.4 ± 6.2*gh

40.5 ± 9.2*gh

34.5 ± 4.4*gh

20.4 ± 5.6*h

29.8 ± 4.2*

 

  DF-Tb.N (mm−1)

4.62 ± 0.53*

2.76 ± 0.34

3.22 ± 0.42*b

2.38 ± 0.35*cdefgh

3.53 ± 0.42*fgh

3.08 ± 0.46*

3.37 ± 0.58*h

2.97 ± 0.22

2.90 ± 0.45*

2.96 ± 0.37*

 

  DF-Tb.Th (μm)

77.7 ± 6.4

70.3 ± 7.0

73.5 ± 6.5bcdefgh

65.7 ± 5.8cdefgh

83.8 ± 7.9*defh

125.1 ± 7.8*gh

122.3 ± 8.6*gh

122.4 ± 8.2*gh

89.1 ± 16.1*h

112.9 ± 5.6*

 

  DF-SMI

0.58 ± 0.60*

1.96 ± 0.45

1.76 ± 0.21*bdefh

2.51 ± 0.36*cdefgh

1.31 ± 0.50*defh

−0.47 ± 0.64*gh

−0.46 ± 0.95*gh

0.07 ± 0.41*gh

1.36 ± 0.59*h

0.58 ± 0.37*

 

  DF-DA

1.41 ± 0.05

1.45 ± 0.06

1.43 ± 0.07defh

1.43 ± 0.09defh

1.54 ± 0.04defh

1.31 ± 0.07*g

1.32 ± 0.07*g

1.40 ± 0.06*g

1.37 ± 0.07h

1.29 ± 0.06*

 

  DF-DBM (mgHA/cm3)

967 ± 20*

953 ± 11

967 ± 22*b

947 ± 19cdefgh

972 ± 13*

990 ± 13*

983 ± 17*

978 ± 10*

983 ± 23*

1,002 ± 18*

 

LV compression

 Number of rats

12

10

12

11

12

12

12

5

12

10

 

  Max load (N)

203 ± 36

220 ± 62

242 ± 54bdefh

178 ± 37cdefgh

213 ± 23defh

357 ± 51*fg

399 ± 112*fgh

299 ± 79*g

243 ± 51h

318 ± 72*

 

  Max stress (MPa)

20.4 ± 2.8

19.9 ± 4.0

20.8 ± 3.5bdefh

17.5 ± 3.0cdefgh

21.5 ± 2.5defh

31.6 ± 4.8*g

34.0 ± 7.7*g

29.7 ± 1.9*g

20.9 ± 3.9h

27.2 ± 4.7*

 

  Yield stress (MPa)

11.8 ± 3.3

13.4 ± 4.8

15.6 ± 4.8defh

13.0 ± 3.4defh

15.7 ± 2.5defh

27.6 ± 5.9*g

26.7 ± 8.3*g

20.1 ± 4.1*g

13.8 ± 4.0h

22.7 ± 5.2*

 

  Stiffness (GPa)

0.54 ± 0.26

0.47 ± 0.15

0.61 ± 0.29*bde

0.48 ± 0.17cdefgh

0.66 ± 0.21*de

0.81 ± 0.23*fgh

1.00 ± 0.40*fgh

0.61 ± 0.17*

0.61 ± 0.19*

0.66 ± 0.19*

 

  Energy absorption (kJ/m2)

4.4 ± 1.8

4.7 ± 2.6

3.9 ± 1.5bdef

7.1 ± 3.4*cgh

5.4 ± 2.8defg

8.0 ± 3.8*gh

7.3 ± 2.4*gh

7.5 ± 1.1*gh

3.7 ± 1.0

6.1 ± 2.5

 

Mean ± SD. Lowercase letters—difference from group (p < 0.05)

LV lumbar vertebral body, DF distal femur, BV/TV bone volume/total tissue volume, Tb.N trabecular number, Tb.Th trabecular thickness, SMI Structure Model Index, DA degree of anisotropy, HA hydroxyapatite, DBM degree of bone mineralization, Max Load maximum load, Max stress maximum stress

*p < .05, difference from OVX

Table 4

Trabecular bone mass, microarchitecture, and bone strength at end of period 2 (day 180)

 

Sham

OVX

Aln-Aln-Aln

Ral-Ral-Ral

Aln-Veh-Aln

PTH-Veh-Veh

PTH-Aln-Veh

PTH-Ral-Ral

Aln-PTH-Veh

Aln-PTH-Aln

Ral-PTH-Ral

Group ID

s

*

a

b

c

d

e

f

g

h

i

MicroCT

 Number of rats

12

10

12

11

12

10

12

12

12

12

11

  LV-BV/TV (%)

40.8 ± 8.5*

18.1 ± 2.5

32.6 ± 3.5*deghi

28.9 ± 3.8*deghi

33.6 ± 3.5*deghi

22.2 ± 1.9efghi

52.0 ± 4.9*f

32.9 ± 3.3*ghi

48.3 ± 3.0*

50.6 ± 4.1*

51.9 ± 6.0*

  LV-Tb.N (mm−1)

3.83 ± 0.37*

2.80 ± 0.25

3.39 ± 0.19*bde

3.11 ± 0.20*ceghi

3.43 ± 0.18*df

2.96 ± 0.17eghi

3.65 ± 0.26*fgh

3.18 ± 0.23*ghi

3.43 ± 0.21*

3.43 ± 0.21*

3.48 ± 0.36*

  LV-Tb.Th (μm)

97.9 ± 15.9*

68.3 ± 3.7

90.0 ± 5.2*deghi

86.6 ± 5.0*deghi

92.4 ± 6.5*deghi

72.3 ± 4.2efghi

133.7 ± 8.5*f

91.5 ± 4.5*ghi

136.2 ± 5.8*

137.0 ± 7.0*

138.2 ± 7.4*

  LV-SMI

−1.67 ± 0.98*

0.61 ± 0.30

−0.53 ± 0.47*deghi

−0.33 ± 0.41*deghi

−0.45 ± 0.39*deghi

0.23 ± 0.17efghi

−2.52 ± 0.70*fgh

−0.70 ± 0.35*ghi

−1.95 ± 0.42*i

−2.15 ± 1.67*i

−2.71 ± 0.85*

  LV-DA

1.67 ± 0.06

1.74 ± 0.07

1.75 ± 0.08h

1.70 ± 0.05

1.75 ± 0.05

1.67 ± 0.06

1.63 ± 0.06

1.68 ± 0.04

1.64 ± 0.07

1.67 ± 0.05

1.64 ± 0.04

  LV-DBM (mgHA/cm3)

934 ± 30*

857 ± 12

892 ± 12*bcdefghi

917 ± 14*di

917 ± 28*dei

860 ± 5efghi

934 ± 17*fi

911 ± 14*i

923 ± 13*i

926 ± 17*i

959 ± 16*

 Number of rats

12

9

12

11

11

10

12

12

12

12

11

  DF-BV/TV (%)

26.1 ± 8.7*

6.2 ± 3.3

13.8 ± 2.3*ceghi

10.2 ± 3.0cefghi

23.0 ± 3.3*defghi

9.7 ± 3.2efghi

35.7 ± 5.5*f

17.5 ± 4.7*ghi

32.3 ± 3.9*i

33.5 ± 4.2*i

34.8 ± 4.3*

  DF-Tb.N (mm−1)

4.23 ± 0.54*

2.09 ± 0.55

2.91 ± 0.33*cdf

2.64 ± 0.24*cegh

3.44 ± 0.35*defi

2.39 ± 0.23*eghi

3.10 ± 0.37*f

2.46 ± 0.44*ghi

3.17 ± 0.30*i

3.17 ± 0.30*i

2.82 ± 0.28*

  DF-Tb.Th (μm)

76.5 ± 11.6*

60.9 ± 4.9

71.6 ± 4.2*cdefghi

66.3 ± 6.6cefghi

84.3 ± 5.9*deghi

62.8 ± 7.0efghi

118.3 ± 8.3*fi

82.9 ± 4.9*ghi

115.8 ± 7.1*i

114.3 ± 5.5*i

128.3 ± 5.2*

  DF-SMI

0.69 ± 0.76*

2.67 ± 0.55

1.98 ± 0.22*cefghi

2.32 ± 0.35cefghi

1.49 ± 0.21*deghi

2.03 ± 0.42*efghi

−0.18 ± 0.48*fgh

1.34 ± 0.40*ghi

0.47 ± 0.32*i

0.36 ± 0.36*i

0.06 ± 0.36*

  DF-DA

1.41 ± 0.07

1.38 ± 0.06

1.44 ± 0.06cdeghi

1.38 ± 0.09ceghi

1.57 ± 0.06*defghi

1.37 ± 0.08eghi

1.28 ± 0.05*f

1.41 ± 0.08ghi

1.29 ± 0.03*

1.31 ± 0.05*

1.30 ± 0.09*

  DF-DBM (mgHA/cm3)

970 ± 19*

954 ± 16

975 ± 10*bdefi

956 ± 15cdeghi

978 ± 10*defi

937 ± 19*efghi

1,004 ± 17*fgh

958 ± 11ghi

985 ± 15*i

981 ± 17*i

997 ± 10*

LV compression

 Number of rats

12

10

12

9

12

11

12

12

12

11

8

  Max load (N)

217 ± 47

174 ± 27

241 ± 44*bcdefghi

188 ± 42eghi

144 ± 39deghi

190 ± 36efghi

366 ± 55*f

171 ± 28ghi

368 ± 87*i

387 ± 50*i

332 ± 28*

  Max stress (MPa)

20.6 ± 4.9*

14.5 ± 1.1

20.9 ± 4.0*cdefgh

17.6 ± 3.5ceghi

11.8 ± 4.3defghi

16.3 ± 2.0eghi

30.5 ± 3.4*f

16.8 ± 2.2ghi

30.5 ± 6.2*

31.7 ± 4.4*

30.3 ± 2.1*

  Yield stress (MPa)

12.1 ± 4.7

12.9 ± 1.8

16.3 ± 6.1cdefgh

12.6 ± 2.6ceghi

6.6 ± 3.4*defghi

11.6 ± 3.9eghi

24.3 ± 4.8*fhi

12.0 ± 1.8ghi

24.2 ± 3.5*i

26.3 ± 5.1*i

19.1 ± 1.6*

  Stiffness (GPa)

0.66 ± 0.27

0.52 ± 0.23

0.68 ± 0.28cde

0.62 ± 0.15ce

0.29 ± 0.13*efghi

0.47 ± 0.15egh

0.89 ± 0.34*fi

0.50 ± 0.17gh

0.76 ± 0.34*

0.86 ± 0.35*

0.68 ± 0.19

  Energy absorption (kJ/m2)

4.4 ± 2.2*

2.1 ± 0.9

3.7 ± 2.3efghi

4.0 ± 2.1efghi

3.9 ± 1.4efghi

3.2 ± 1.5efghi

7.2 ± 2.6*hi

7.9 ± 2.2*hi

5.8 ± 2.4*i

6.9 ± 3.6*i

10.1 ± 4.7*

Mean ± SD. Lowercase letters—difference from group (p < .05)

LV lumbar vertebral body, DF distal femur, BV/TV bone volume/total tissue volume, Tb.N trabecular number, Tb.Th trabecular thickness, SMI Structure Model Index, DA degree of anisotropy, HA hydroxyapatite, DBM degree of bone mineralization, Max load maximum load, Max stress maximum stress

*p < .05, difference from OVX

Table 5

Trabecular bone mass, microarchitecture, and bone strength at the end of period 3 (day 270)

 

Sham

OVX

Aln-Aln-Aln

Ral-Ral-Ral

Aln-Veh-Aln

PTH-Veh-Veh

PTH-Aln-Veh

PTH-Ral-Ral

Aln-PTH-Veh

Aln-PTH-Aln

Ral-PTH-Ral

Group ID

s

*

a

b

c

d

e

f

g

h

i

MicroCT

 Number of rats

7

9

12

12

15

8

6

12

12

9

11

  LV-BV/TV (%)

42.5 ± 9.6*

20.6 ± 3.5

33.0 ± 2.5*bdeghi

27.2 ± 2.9*cdefghi

31.8 ± 4.1*deghi

20.3 ± 3.3efghi

46.2 ± 2.7*fghi

32.8 ± 4.5*ghi

40.0 ± 3.9*h

54.4 ± 4.7*i

39.3 ± 6.5*

  LV-Tb.N (1/mm)

3.93 ± 0.41*

2.78 ± 0.27

3.34 ± 0.15*bdh

3.00 ± 0.27cdeghi

3.38 ± 0.25*dfh

2.65 ± 0.22efghi

3.53 ± 0.13*fh

3.16 ± 0.27*gh

3.48 ± 0.28*h

3.85 ± 0.30*i

3.26 ± 0.29*

  LV-Tb.Th (μm)

98.9 ± 15.8*

71.0 ± 4.3

89.7 ± 5.5*deghi

84.2 ± 2.9*defghi

89.1 ± 4.4*deghi

73.2 ± 4.2efghi

116.3 ± 4.4*fgh

93.3 ± 7.3*ghi

103.8 ± 5.8*h

132.4 ± 7.4*i

109.3 ± 13.4*

  LV-SMI

−1.97 ± 1.02*

0.14 ± 0.31

−0.65 ± 0.26*degh

−0.30 ± 0.31eghi

−0.32 ± 0.39deghi

0.23 ± 0.28efghi

−1.96 ± 0.35*fghi

−0.71 ± 0.36*fg

−1.33 ± 0.38*h

−3.02 ± 0.83*i

−1.17 ± 0.62*

  LV-DA

1.65 ± 0.03

1.71 ± 0.06

1.77 ± 0.05

1.70 ± 0.04

1.78 ± 0.06

1.68 ± 0.05

1.70 ± 0.04

1.66 ± 0.04

1.67 ± 0.04

1.69 ± 0.06

1.65 ± 0.06

  LV-DBM (mgHA/cm3)

928 ± 25*

886 ± 18

931 ± 16*bcdeh

908 ± 18*eghi

912 ± 16*degh

890 ± 11efghi

955 ± 26*fi

918 ± 20*gh

937 ± 15*h

970 ± 23*i

927 ± 22*

 Number of rats

7

10

11

12

15

11

11

11

12

10

11

  DF-BV/TV (%)

25.4 ± 10.5*

6.8 ± 1.9

14.4 ± 3.2*bcdeghi

10.0 ± 2.0cefghi

18.9 ± 3.2*deghi

7.6 ± 2.3efghi

26.5 ± 3.7*fh

16.7 ± 2.7*ghi

23.5 ± 6.5*h

36.0 ± 5.0*i

27.8 ± 7.8*

  DF-Tb.N (1/mm)

4.17 ± 0.76*

2.28 ± 0.24

2.91 ± 0.30*bdfh

2.42 ± 0.33cegh

3.23 ± 0.34*dfi

2.11 ± 0.32efghi

2.93 ± 0.32*fh

2.51 ± 0.25gh

2.99 ± 0.56*h

3.42 ± 0.23*i

2.75 ± 0.43*

  DF-Tb.Th (μm)

75.5 ± 15.7*

64.0 ± 4.3

73.1 ± 3.6*eghi

72.7 ± 4.0*cefghi

80.3 ± 5.5*deghi

66.4 ± 7.5efghi

97.7 ± 7.8*fhi

80.7 ± 4.1*ghi

91.8 ± 7.4*hi

115.3 ± 10.5*

110.6 ± 23.5*

  DF-SMI

0.74 ± 0.80*

2.64 ± 0.31

2.00 ± 0.29*efghi

2.38 ± 0.25cefghi

1.85 ± 0.26*defghi

2.38 ± 0.44efghi

0.68 ± 0.34*fh

1.38 ± 0.24*hi

1.04 ± 0.48*hi

0.07 ± 0.49*i

0.50 ± 0.65*

  DF-DA

1.41 ± 0.06

1.37 ± 0.08

1.47 ± 0.06*bdeghi

1.41 ± 0.07ch

1.50 ± 0.04*deghi

1.41 ± 0.12gh

1.35 ± 0.05f

1.46 ± 0.06*ghi

1.35 ± 0.07

1.31 ± 0.07

1.35 ± 0.07

  DF-DBM (mgHA/cm3)

968 ± 19

956 ± 12

982 ± 11*bdfh

968 ± 10cdehi

987 ± 13*dfg

953 ± 16eghi

984 ± 13*f

961 ± 8hi

971 ± 15*h

997 ± 22*

989 ± 31*

LV compression

 Number of rats

7

10

12

12

15

11

11

11

12

10

9

  Max load (N)

255 ± 68*

159 ± 55

227 ± 51*bcdegh

162 ± 21eghi

171 ± 47eghi

154 ± 38efghi

282 ± 45*fhi

205 ± 43gh

275 ± 51*hi

387 ± 68*i

225 ± 51*

  Max stress (MPa)

21.9 ± 6.0*

13.2 ± 4.1

19.1 ± 3.4*bcdegh

15.5 ± 2.3efghi

15.2 ± 4.3efghi

13.0 ± 3.3efghi

23.6 ± 3.2*fhi

20.2 ± 4.0*h

22.7 ± 3.4*h

31.6 ± 3.8*i

19.8 ± 4.5*

  Yield stress (MPa)

15.5 ± 4.1*

9.4 ± 3.3

15.2 ± 3.1*bcdh

10.8 ± 2.3cefghi

7.3 ± 2.0efghi

10.0 ± 4.1efghi

15.3 ± 3.0*h

15.2 ± 3.9*h

17.5 ± 4.2*h

23.8 ± 6.1*i

15.3 ± 4.8*

  Stiffness (GPa)

0.73 ± 0.24*

0.45 ± 0.22

0.63 ± 0.22cdh

0.44 ± 0.14fh

0.37 ± 0.13fgh

0.43 ± 0.20fgh

0.53 ± 0.16fh

0.73 ± 0.27*

0.62 ± 0.16h

0.86 ± 0.25*i

0.55 ± 0.22

  Energy absorption (kJ/m2)

4.5 ± 1.8

2.9 ± 2.7

3.3 ± 1.3behi

5.3 ± 1.7*cdi

3.2 ± 2.0ehi

2.3 ± 0.9efghi

7.1 ± 2.3*fg

4.5 ± 2.2hi

4.8 ± 2.2i

6.5 ± 2.0*

8.0 ± 2.9*

Mean ± SD. Lowercase letters—difference from group (p < .05)

LV lumbar vertebral body, DF distal femur, BV/TV bone volume/total tissue volume, Tb.N trabecular number, Tb.Th trabecular thickness, SMI Structure Model Index, DA degree of anisotropy, HA hydroxyapatite, DBM degree of bone mineralization, Max load maximum load, Max stress maximum stress

*p < .05, difference from OVX

Table 6

Data from Combinable Groups in periods 1 and 2

Variable

Aln-Aln-Aln, Aln-Veh-Aln, Aln-PTH-Veh, and Aln-PTH-Aln (period 1)

PTH-Veh-Veh, PTH-Aln-Veh, and PTH-Ral-Ral (period 1)

Aln-PTH-Veh and Aln-PTH-Aln (period 2)

Ral-Ral-Ral and Ral-PTH-Ral (period 1)

MicroCT

 Number of rats

47

30

24

11

  LV-BV/TV (%)

37.9 ± 8.5

51.6 ± 6.9

47.0 ± 4.9

27.8 ± 2.4

  LV-Tb.N (mm−1)

3.48 ± 0.25

3.52 ± 0.21

3.32 ± 0.26

3.19 ± 0.17

  LV-Tb.Th (μm)

102.2 ± 19.4

133.8 ± 14.2

135.6 ± 6.2

81.7 ± 4.6

  LV-SMI

−1.05 ± 0.92

−2.48 ± 0.93

−1.60 ± 0.73

−0.29 ± 0.28

  LV-DA

1.73 ± 0.08

1.68 ± 0.09

1.66 ± 0.07

1.71 ± 0.04

  LV-DBM (mgHA/cm3)

912 ± 34

929 ± 23

924 ± 15

904 ± 17

 Number of rats

45

29

24

11

  DF-BV/TV (%)

22.5 ± 6.8

39.0 ± 7.5

32.9 ± 4.0

8.5 ± 2.8

  DF-Tb.N (mm−1)

3.17 ± 0.48

3.18 ± 0.50

3.17 ± 0.30

2.38 ± 0.35

  DF-Tb.Th (μm)

88.8 ± 17.1

123.4 ± 8.0

115.0 ± 6.2

65.7 ± 5.8

  DF-SMI

1.29 ± 0.58

−0.37 ± 0.76

0.41 ± 0.34

2.51 ± 0.36

  DF-DA

1.41 ± 0.11

1.33 ± 0.07

1.30 ± 0.04

1.43 ± 0.09

  DF-DBM (mgHA/cm3)

980 ± 23

985 ± 15

983 ± 17

947 ± 19

LV compression

 Number of rats

46

29

23

11

  Max load (N)

251 ± 62

364 ± 91

377 ± 71

178 ± 37

  Max stress (MPa)

22.4 ± 4.4

32.3 ± 5.9

31.1 ± 5.3

17.5 ± 3.0

  Yield stress (MPa)

16.7 ± 5.2

25.9 ± 7.1

25.2 ± 4.4

13.0 ± 3.4

  Stiffness (GPa)

0.63 ± 0.22

0.86 ± 0.33

0.81 ± 0.34

0.48 ± 0.17

  Energy absorption (kJ/m2)

4.73 ± 2.06

7.60 ± 2.85

6.32 ± 3.02

7.08 ± 3.38

Mean ± SD

LV lumbar vertebral body, DF distal femur, BV/TV bone volume/total tissue volume, Tb.N trabecular number, Tb.Th trabecular thickness, SMI Structure Model Index, DA degree of anisotropy, HA hydroxyapatite, DBM degree of bone mineralization, Max load maximum load, Max stress maximum stress

The relationship of BV/TV to SMI and DA in both the vertebral body and DFM was evaluated using linear regression. The bone mass, cross-sectional area, and microarchitectural endpoints that determine vertebral body maximum load were evaluated with multiple regression. All statistical analyses were performed with SAS v9.2 (Cary, NC, USA).

Results

The uteri in OVX rats were routinely observed to be smaller than those of sham rats at necropsy. Compared with sham, OVX resulted in a significant increase in body weight within 4 weeks of surgery (p < 0.05). However, once pair feeding was begun, body weight in Veh-Veh-Veh rats dropped gradually and was not significantly different from sham rats at the end of any treatment period. Body weight was not affected by any treatment (data not shown).

BV and microarchitecture

Fifth lumbar vertebral body

At baseline and all subsequent time periods, lumbar vertebral body bone volume (LV-BV/TV) in Veh-Veh-Veh was less than in sham rats (Tables 2, 3, 4, and 5; Fig. 1a). At day 90, all treatment groups except for Ral-Ral-Ral, had higher LV-BV/TV than Veh-Veh-Veh (Table 3). Of the four groups that received Aln monotherapy in period 1, three (Aln-Aln-Aln, Aln-Veh-Aln, and Aln-PTH-Veh) had LV-BV/TV ranging from 32 to 36 %, whereas the fourth (Aln-PTH-Aln) had LV-BV/TV of 51 %, with a range in individual rats of 42–58 %, which was significantly different from the other three Aln-first groups (Fig. 1c). At day 180, LV-BV/TV in PTH-Veh-Veh was the same as Veh-Veh-Veh, whereas all other treatment groups were above Veh-Veh-Veh (Table 4; Fig. 1b). LV-BV/TV in PTH-Aln-Veh was higher than in all other treatment groups. By day 270, all regimens that included PTH, except for PTH-Veh-Veh and PTH-Ral-Ral, had greater LV-BV/TV than the other treatment groups (Table 5; Fig. 1b, c).
Fig. 1

These figures provide a graphical explanation of the study design and general outcome. The complete dataset with statistical testing is reported in Tables 2, 3, 4, and 5. The data represent individual groups of rats at each time period rather than longitudinal measurement of one set of rats. a LV-BV/TV data for all groups that received only antiresorptive monotherapy (A-A-A, R-R-R, and A-V-A) are shown. Sham and Veh-Veh-Veh group data are included for comparison. Veh-Veh-Veh rats had LV-BV/TV at ∼30 % lower than that in sham rats at day 0, signaling established estrogen-deficiency osteopenia as treatments began. By day 270, osteopenia in Veh-Veh-Veh groups had progressed. Ral-Ral-Ral-treated groups did not develop additional osteopenia after day 90. Both Aln-Aln-Aln and Aln-Veh-Aln rats had significantly higher LV-BV/TV throughout the experiment than either Ral-Ral-Ral or Veh-Veh-Veh rats but never reached levels in age-matched sham rats. b LV-BV/TV data for all groups that received PTH(1–34) during period 1 (P-V-V, P-A-V, and P-R-R) are shown, along with sham and Veh-Veh-Veh data. All groups experienced a rapid increase in LV-BV/TV by day 90, which exceeded the sham levels, as PTH was given. Failure to follow up PTH cessation with antiresorptive therapy was associated with a rapid decline in LV-BV/TV, which reached Veh-Veh-Veh levels by day 180. Following up PTH cessation with Ral for 180 days was associated with significantly lower LV-BV/TV, which did not, however, reach the Veh-Veh-Veh level by day 270. Following up PTH cessation with Aln was associated with stable LV-BV/TV for 90 days. However, when Aln was stopped, a slow decline began that did not reach Veh-Veh-Veh levels by day 270 and was superior to levels seen with Ral maintenance. c LV-BV/TV data for groups that began with antiresorptive monotherapy, then switched to PTH (A-P-V, A-P-A, and R-P-R), are shown with sham and Veh-Veh-Veh data. The mean of all four groups that received ALN during period 1 is also plotted in gray. All treated groups experienced a rapid increase in LV-BV/TV between days 90 and 180, which exceeded sham levels, as PTH was administered. After PTH cessation, switching rats to Aln was significantly better than either switching to Ral or stopping all treatment. Unlike when PTH was given to treatment-naïve rats in period 1, when PTH was given to rats pretreated with antiresorptive monotherapy and then discontinued without follow-up treatment, LV-BV/TV did not return to Veh-Veh-Veh levels within 90 days

At all times, lumbar vertebral body trabecular number (LV-Tb.N) in Veh-Veh-Veh was less than in sham rats (Tables 2, 3, 4, and 5). At all times except day 90, lumbar vertebral body trabecular thickness (LV-Tb.Th) in Veh-Veh-Veh was less than in sham rats (Tables 2, 3, 4, and 5). At day 90, all treatment groups except for Ral-Ral-Ral, had higher Tb.N and Tb.Th than Veh-Veh-Veh (Table 3). Of the four groups that received Aln monotherapy in period 1, three (Aln-Aln-Aln, Aln-Veh-Aln, and Aln-PTH-Veh) had LV-Tb.Th that ranged from 88 to 97 μm, whereas the fourth (Aln-PTH-Aln) had LV-Tb.Th of 132 μm, with a range in individual rats of 125–141 μm, which was significantly different from the other three Aln-first groups. At day 180, all groups except for PTH-Veh-Veh had higher LV-Tb.N and LV-Tb.Th than Veh-Veh-Veh (Table 4). By day 270, all regimens that included PTH, except for PTH-Veh-Veh and PTH-Ral-Ral, had greater LV-Tb.N, and LV-Tb.Th than other groups (Table 5).

At all times, lumbar vertebral body degree of bone mineralization (LV-DBM) in Veh-Veh-Veh was less than in sham rats (Tables 2, 3, 4, and 5). At day 90, all groups except for Aln-Aln-Aln, Aln-Veh-Aln, Aln-PTH-Veh, and Ral-Ral-Ral had higher LV-DBM than Veh-Veh-Veh (Table 3). At day 180, LV-DBM in PTH-Veh-Veh was the same as Veh-Veh-Veh, whereas all other treatment groups were above Veh-Veh-Veh (Table 4). LV-DBM in Ral-PTH-Ral was higher than all other treatment groups (Table 4). By day 270, all regimens that included both PTH and Aln had greater LV-DBM than other treated groups, except for Ral-PTH-Ral and Aln-Aln-Aln (Table 5).

At all times, lumbar vertebral body Structure Model Index (LV-SMI) in Veh-Veh-Veh was higher than in sham rats (Tables 2, 3, 4, and 5). At day 90, though all PTH treatment groups had lower SMI than Veh-Veh-Veh, antiresorptive treatment alone did not affect SMI (Table 3). Of the four groups that received Aln monotherapy in period 1, three (Aln-Aln-Aln, Aln-Veh-Aln, and Aln-PTH-Veh) had LV-SMI that ranged from −0.48 to −0.9, whereas the fourth (Aln-PTH-Aln) had LV-SMI of −2.43, with a range in individual rats of −1.45 to −3.51, which was significantly different from the other three Aln-first groups. By day 180, LV-SMI in PTH-Veh-Veh was the same as Veh-Veh-Veh, whereas all other groups were significantly below Veh-Veh-Veh. In addition, LV-SMI in the antiresorptive only groups is significantly greater than LV-SMI in all groups that received PTH, except PTH-Ral-Ral (Table 4). At day 270, PTH-Veh-Veh was the only group whose LV-SMI did not differ from Veh-Veh-Veh. Antiresorptive only groups were significantly below Veh-Veh-Veh, whereas groups that combined PTH with an antiresorptive had the lowest values for LV-SMI and were significantly lower than LV-SMI in the antiresorptive only groups. The lowest value was obtained in Aln-PTH-Aln at day 270 (Table 5).

Lumbar vertebral body degree of anisotropy (LV-DA) in Veh-Veh-Veh differed from sham only at day 90 (Table 2). There were no differences in LV-DA among all treatment groups at any time (Tables 3, 4, and 5).

Distal femoral metaphysis

At all times, distal femur bone volume (DF-BV/TV) in Veh-Veh-Veh was less than in sham rats (Tables 2, 3, 4, and 5). At day 90, all treatment groups except for Aln-Aln-Aln and Ral-Ral-Ral had higher DF-BV/TV than Veh-Veh-Veh (Table 3). At day 180, DF-BV/TV in PTH-Veh-Veh and Ral-Ral-Ral were not different from Veh-Veh-Veh, whereas all the other treatment groups were above Veh-Veh-Veh (Table 4). PTH-Aln-Veh, Aln-PTH-Veh, Aln-PTH-Aln, and Ral-PTH-Ral had greater DF-BV/TV than antiresorptive only groups (Table 4). By day 270, all regimens that included PTH, except for PTH-Veh-Veh, had greater DF-BV/TV than other treatment groups. Aln-PTH-Aln had greater DF-BVTV than any other group (Table 5).

At all times, distal femur trabecular number (DF-Tb.N) in Veh-Veh-Veh was less than in sham rats (Tables 2, 3, 4, and 5). At day 90, all groups except for Ral-Ral-Ral had higher DF-Tb.N than Veh-Veh-Veh (Table 3). Furthermore, though all treatment regimens except Aln-Aln-Aln and Ral-Ral-Ral had higher DF-Tb.Th than Veh-Veh-Veh (Table 3), the greatest positive differences were seen in the PTH-treated groups that were greater than antiresorptive alone groups. By day 180, all treatment regimens had higher DF-Tb.N than Veh-Veh-Veh. All treatment regimens that included PTH except for PTH-Veh-Veh and PTH-Ral-Ral had higher DF-Tb.Th than Veh-Veh-Veh (Table 4). Antiresorptive groups that include Aln, but not Ral had significantly higher DF-Tb.Th than Veh-Veh-Veh. By day 270, Aln-Veh-Aln plus all regimens that included PTH, except for PTH-Veh-Veh and PTH-Ral-Ral had greater DF-Tb.N than other groups. By day 270, all regimens that included PTH, except for PTH-Veh-Veh and Ral-Ral-Ral, also had greater DF-Tb.Th than other treatment groups. Treatment groups that included both PTH and Aln, plus Ral-PTH-Ral, had the highest DF-Tb.N values (Table 5).

Before day 270, distal femur degree of bone mineralization (DF-DBM) in Veh-Veh-Veh was less than in sham rats (Tables 2, 3, 4, and 5). At day 90, all treatment groups except Ral-Ral-Ral had higher DF-DBM than Veh-Veh-Veh (Table 3). At day 180, DF-DBM in PTH-Veh-Veh, PTH-Ral-Ral and Ral-Ral-Ral was the same as Veh-Veh-Veh, whereas all other groups were greater than Veh-Veh-Veh (Table 4). DF-DBM in PTH-Aln-Veh was higher than all other treatment groups (Table 4). By day 270, PTH-Aln-Veh, Aln-Aln-Aln, Aln-Veh-Aln, Aln-PTH-Aln and Ral-PTH-Ral had greater DF-DBM than other groups (Table 5).

At all times, distal femur Structure Model Index (DF-SMI) in Veh-Veh-Veh was higher than in sham rats (Tables 2, 3, 4, and 5). At day 90, all treatment groups except for Aln-Aln-Aln and Ral-Ral-Ral had a lower DF-SMI than Veh-Veh-Veh. Groups that involved PTH treatment had the lowest DF-SMI values (Table 3). At day 180, all groups except for Ral-Ral-Ral had lower DF-SMI than Veh-Veh-Veh. The lowest DF-SMI values were in groups with both PTH and antiresorptive treatment (Table 4). By day 270, PTH-Veh-Veh and Ral-Ral-Ral no longer differed from Veh-Veh-Veh. The lowest values were found in groups that had received both PTH and antiresorptive treatment (Table 5).

Distal femur degree of anisotropy (DF-DA) in Veh-Veh-Veh did not differ from sham at any time (Table 2, 3, 4, and 5). At day 90, DF-DA in all regimens that included PTH was lower than the Veh-Veh-Veh group (Table 3). At day 180, DF-DA in PTH-Aln-Veh, Aln-PTH-Veh and Aln-PTH-Aln was less than Veh-Veh-Veh (Table 4). DF-DA in Aln-Veh-Aln was higher than all other groups. At day 270, Aln-Veh-Aln had greater DF-DA than all other groups (Table 5).

Mechanical testing (LV6)

At baseline and day 90, maximum load, maximum stress and yield stress in Veh-Veh-Veh did not differ from sham (Tables 2 and 3). At day 180, maximum stress in Veh-Veh-Veh was lower than in sham rats (Table 4). By day 270, maximum load, maximum stress, and yield stress were all lower in Veh-Veh-Veh than sham (Table 5). At day 90, all PTH regimens and Aln-PTH-Aln had higher maximum load, maximum stress, and yield stress than Veh-Veh-Veh. Of the four groups that received Aln monotherapy in period 1, three (Aln-Aln-Aln, Aln-Veh-Aln, and Aln-PTH-Veh) had maximum load that ranged from 213 to 243 N, whereas the fourth (Aln-PTH-Aln) had maximum load 318 N, with a range in individual rats of 217–473, that was significantly different from the other three Aln-first groups. The maximum load for the combined groups was 251 ± 62 N (Table 6). By day 180, all PTH regimens, except PTH-Veh-Veh and PTH-Ral-Ral, had greater maximum load, maximum stress, and yield stress than the other groups. Though Aln-Veh-Aln strength did not different from Veh-Veh-Veh, all other groups that received Aln had maximum load, maximum stress and yield stress greater than Veh-Veh-Veh (Table 4). By day 270, Aln-Aln-Aln and all PTH-containing regimens, except for PTH-Veh-Veh had greater maximum load, maximum stress, and yield stress than other groups. Aln-PTH-Aln had the highest maximum load, maximum stress, and yield stress of all the groups (Table 5).

Intrinsic values for 3D microarchitectural variables

SMI and DA were measured in trabecular bone regions from over 350 samples each from the LV5 body and DFM. BV/TV ranged from 2.4 to 61.9 %. SMI ranged from −4.74 to +3.31 and DA ranged from 1.185 to 1.913. SMI was inversely and very strongly correlated to BV/TV (r = −0.954) (Fig. 2a). DA was about 20 % greater in the vertebral body than the DFM, and was weakly correlated to BV/TV in both sites (r = −0.289 and −0.346) (Fig. 2b).
Fig. 2

a Data describing the relationship of SMI to BV/TV in 752 samples are plotted. Data from the lumbar vertebral body and DFM are plotted in closed or open triangles, respectively. Many specimens exhibit a negative SMI, particularly those with BV/TV greater than 0.3. There is a very strong negative linear relationship of SMI to BV/TV, whether considering all samples (r = 0.954, solid line), vertebral body only (r = 0.972, dotted line), or DFM only (r = 0.971, dashed line). BV/TV explains over 91 % of the variation in SMI in the whole dataset. The y-intercepts for the vertebral body and distal femur differ, indicating that the vertebral body has a significantly more negative SMI for any BV/TV value than the DFM. b Data describing the relationship of DA to BV/TV in the vertebral body and DFM, respectively, are plotted as closed or open triangles. The vertebral body and DFM data are distinct populations, with the vertebral body being ∼20 % higher than the distal femur. There is a weak negative linear relationship of DA to BV/TV in both the vertebral body only (r = 0.289, dotted line) and DFM only (r = 0.35, dashed line). The y-intercepts for the vertebral body and distal femur differ, indicating that the vertebral body has a significantly higher DA for any BV/TV value than the DFM

Relationship of vertebral body maximum load to measurable vertebral body surrogate bone endpoints

Lumbar vertebral BV/TV, Tb.Th, and SMI were well-correlated to vertebral body maximum load, each explaining at least 55 % of the variation in maximum load (Table 7; Fig. 3). However, they were correlated to each other to such an extent that, in multiple regression (Table 8), Tb.Th, followed by cross-sectional area, with small contributions from DBM and SMI had the strongest association with maximum load. Together, these four endpoints explained 78 % of the variation in maximum load (Table 8). The correlation coefficient of maximum load to DA (r = −0.26) had a smaller absolute value than the correlation coefficients of maximum load to Tb.N, SMI, BV/TV, and Tb.Th, respectively (r = 0.37–0.81) (Table 7).
Table 7

Correlation coefficients of maximum load to measurable bone endpoints in the lumbar vertebral body

Endpoint

R

BV/TV

0.768

Tb.Th

0.812

Tb.N

0.370

SMI

−0.742

DA

−0.263

DBM

0.477

Cross-sectional area

0.443

BV/TV bone volume/tissue volume, Tb.Th trabecular thickness, Tb.N trabecular number, SMI Structure Model Index, DA degree of anisotropy, DBM degree of bone mineralization

Fig. 3

Data describing the relationship of maximum load to Tb.Th in the vertebral body are plotted. Non-PTH- vs. PTH-treated rats killed immediately at the end of therapy or after antiresorptive maintenance are shown as open or closed circles, respectively. These data indicate a very strong relationship of Tb.Th to vertebral body maximum load, which is driven by treatment with PTH, producing trabecular bone regions with thicker trabeculae and lower SMI than in non-PTH-treated rats

Table 8

Determinants of maximum load (multiple regression) in the lumbar vertebral body

Endpoint

Total R

Tb.Th

0.812

Cross-sectional area

0.860

DBM

0.873

SMI

0.882

Tb.Th trabecular thickness, DBM degree of bone mineralization, SMI Structure Model Index

Discussion

Osteoporosis often requires medical treatment that continues for many years. No single currently approved medication begins to approach complete elimination of fracture risk. Fortunately, approved agents with complementary mechanisms of action are available. One could theorize that the systematic, sequential application of existing approved medicines that act strongly with complementary tissue-level actions can provide better fracture risk reduction than any single monotherapy.
Table 9

Correlation of microarchitectural endpoints of vertebral body to distal femoral metaphysis

Endpoint

R

Tb.Th

0.92

Tb.N

0.76

BV/TV

0.89

SMI

0.84

Tb.Th trabecular thickness, Tb.N trabecular number, BV/TV bone volume/tissue volume, SMI Structure Model Index

We investigated the effects of sequential treatment of osteopenic, OVX rats with complementary anti-osteoporosis agents on bone mass, bone microarchitecture, and bone strength endpoints. Each agent is efficacious by itself in reducing vertebral fracture risk in humans [2, 23, 24]. The first two categories of endpoints are generally known to be related to bone strength and provide a nondestructive, measurable surrogate for risk of fracture in humans. We examined all three categories of endpoints in OVX rats treated with each of the three agents as monotherapy. At the same time, we tested the monotherapies head to head against a series of sequences that have been or may be used in humans, again assessing the three categories of endpoints.

Our study started with 6-month-old OVX rats and allowed 8 weeks for development of estrogen-deficient osteopenia. Our data (Tables 2, 3, 4, and 5) show that estrogen-deficiency bone loss continued throughout the first two 3-month-treatment periods. As there was ongoing bone loss to prevent, this was a favorable environment for observing efficacy of antiresorptive monotherapy [21].

Physicians prescribe antiresorptive treatment as initial monotherapy for osteoporotic patients, because this treatment is proven to reduce fracture risk, and is safe, convenient, and cost-effective [23]. Accordingly, several treatment groups addressed such therapy and one even included a bisphosphonate “treatment holiday.” Rats given continuous treatment with a bisphosphonate had better bone strength with modestly better bone mass and trabecular microarchitecture than untreated OVX rats. Though both the bisphosphonate drug holiday and continuous Ral groups had better bone mass, Tb.N, and Tb.Th than untreated OVX rats, these findings were not associated with better bone strength. As in humans, bisphosphonate treatment was associated with lower SMI [25]. Neither DBM nor SMI were as good in drug holiday and continuous Ral groups, as in the continuous Aln group. This may imply that the demonstrated relationship of DBM and SMI to bone strength (Table 7) is reflected in a practical way in the drug holiday and continuous Ral groups. Considering that bisphosphonate treatment increases DBM above that found in osteopenic subjects [26], some hypothesize that higher DBM accounts for a portion of the increased BMD and reduced fracture risk seen with antiresorptives [27]. Continuous antiresorptive therapy also improves the degree and uniformity of bone mineralization and maintains trabecular microstructure [28]. Our results for continuous Aln agree, showing better bone strength than in untreated OVX rats.

Clinical studies of fracture risk that include a bisphosphonate drug holiday have reported that fracture risk may rise, leading to recommendations that drug holidays be limited to patients with low or moderate fracture risk [29] and then only with intermittent monitoring for bone loss or fracture [30]. Our results show that despite better bone mass and microarchitecture in the drug holiday group than in untreated OVX rats, bone strength is not better. These preclinical data agree with reports that drug “holidays” may be associated with less antifracture efficacy than continuous bisphosphonate use. Our data also suggest that continuous bisphosphonate use provides better antifracture efficacy than continuous Ral. Though we increased our Ral dose by 150 % over one that was efficacious in a previous experiment [31], we cannot rule out that alternate day dosing of Ral may have contributed to this finding. It is known that 1.5 mg kg−1 dose−1 daily Ral by oral gavage is the minimum dose that is sufficient to prevent most OVX-induced bone loss in adult rats [20].

Osteopenic, OVX rats treated with PTH had higher bone mass, better microarchitecture, and greater bone strength than untreated OVX rats at the end of PTH treatment. In particular, Tb.Th was 45–78 % higher and Tb.N was 8–22 % higher after PTH treatment, based on data from LV5 and the distal femur. However, these anabolic effects were completely lost within 3 months of PTH discontinuation, leaving the bone the same as untreated OVX rats, consistent with multiple preclinical and clinical studies [7, 32]. For example, PTH cessation in OVX rats caused trabecular bone loss that is detectable by 4 weeks with almost complete disappearance by 12 weeks [32]. Significant BMD loss is seen in postmenopausal women who receive placebo for 1 year after cessation of PTH [7].

PTH therapy is only approved by the FDA for a duration of 24 months [6]. For an individual patient, it is a matter of when, not if, PTH is discontinued. In agreement with other reports [7, 12, 13], our data show that post-PTH medical treatment must be provided to avoid loss of PTH’s therapeutic benefit. Our study demonstrates that rats given PTH monotherapy followed by Aln monotherapy had better bone mass, microarchitecture, and strength than rats in which PTH was stopped without a follow-up treatment. These results are similar to those reported in clinical studies of PTH followed by a bisphosphonate or Ral [7, 12, 13]. We found that Aln was more efficacious than Ral in preserving PTH-related improvements in bone mass, microarchitecture, and strength. As previously mentioned, this may be related to our Ral dosing regimen. Other preclinical studies have investigated sequential therapies in OVX rats in which PTH was followed by estrogen [16], zoledronic acid [14], or risedronate [19]. These follow-up treatments resulted in lack of maintenance by estrogen replacement or maintenance with further gain in bone mass and strength with the bisphosphonates. Though the different times allowed from OVX to treatment initiation may play a role in the different findings, the strength of the antiresorptives employed may also be responsible. The data suggest that bisphosphonates are generally more efficacious at preserving PTH-related gains in bone strength than estrogen receptor binding antiresorptives. Our data indicate that initiating osteoporosis treatment with an anabolic agent then following up with bisphosphonate maintenance, could lead to better fracture risk reduction than either antiresorptive monotherapy or PTH treatment without follow-up maintenance.

In clinical practice, patients who fracture or lose bone while being treated with one medication require other therapeutic options. Preclinical data may guide those choices. We evaluated three groups that started with first-line therapy, then switched to PTH. Two of the three subsequently continued with the types of post-PTH maintenance tested previously here and discussed above.

All groups that switched to PTH from antiresorptive monotherapy had higher bone mass, higher Tb.Th, lower SMI, and higher bone strength immediately after the conclusion of treatment, than both untreated OVX rats and rats receiving continuous antiresorptives. The values were similar to those in rats that began PTH without prior use of antiresorptives, especially when considering the means of the combined groups with like treatment (Table 6). This indicates that prior antiresorptive treatment did not interfere with the skeleton’s ability to respond positively to PTH, in agreement with previous preclinical findings [33].

Rats treated with PTH that was followed by no maintenance treatment, had lower bone mass, microarchitecture, and bone strength than rats that were switched to Aln maintenance therapy at the end of PTH treatment. Rats that switched to Ral maintenance had bone mass, microarchitecture, and bone strength similar to rats that had no post-PTH maintenance treatment. Another preclinical study that evaluated PTH combined with a Ral analog to treat osteopenia in OVX rats reported significant reduction in bone mass at several skeletal sites during Ral maintenance [34]. Though our results agree with other reports [34] that suggest a bisphosphonate is a better post-PTH maintenance therapy than Ral, we cannot rule out that our Ral dosing regimen influenced this finding.

At the end of study, rats treated sequentially with Aln, then PTH, and then Aln had the highest bone mass, microarchitecture, and bone strength of all the groups. These endpoints were at least as high as, and often higher than, sham rats. The Aln-PTH-Aln group at day 90 was significantly different from the other three day 90 groups treated with Aln for vertebral body endpoints. However, these are not the same rats that comprise the days 180 and 270 Aln-PTH-Aln groups. Therefore, these results need to be evaluated at the end of each treatment period (e.g., 90, 180, or 270 days) and not across the treatment periods. Our preclinical data suggest that a treatment plan that starts with a bisphosphonate, then switches to a period of PTH that significantly improves bone mass and microarchitecture, then switches to long-term bisphosphonate maintenance, is likely to have greater antifracture efficacy than any monotherapy. These results support the current choice of firstline therapy as antiresorptives, because one would predict that combining such therapy sequentially with an anabolic agent as needed, can provide more fracture risk reduction than can be achieved with either continuous frontline therapy or starting with PTH and then maintaining.

Our results provide additional practical data about SMI, an endpoint that describes the rod- or plate-like nature of trabecular lattices [35]. SMI has been thought to range mainly from zero to three, more negative values representing a more plate-like, stronger lattice [36, 37]. We found that, as in humans [38], PTH treatment reduced SMI. Though SMI here was very strongly and inversely correlated to BV/TV (Fig. 2a) [39, 40], almost half the specimens had negative SMI [36]. However, most existing SMI studies characterize samples from normal and osteopenic subjects with BV/TV in the range of 2–25 % [41] and report positive SMI [39, 42, 43, 44]. By contrast, one third of our specimens were from rats that had received efficacious treatment with anti-osteoporosis agents, which had BV/TV greater than 35 %. BV/TV was greater and SMI was more negative in the lumbar vertebral body than the DFM, but the values of the two bone sites were still highly correlated (Tables 2--5 and 9). Negative SMI indicates pores within high BV/TV trabecular lattices that have a structure with a concave surface [37]. Negative SMI has been seen in the proximal tibial epiphysis where BV/TV is 35–40 %, vs. the neighboring metaphysis where SMI is positive and BV/TV is 10–30 % [40]. The current data indicate that when specimens with high BV/TV are evaluated, SMI can be negative. SMI is an important determinant of bone strength (Table 7) [44, 45] and fracture risk [46], negative values being associated with the best strength.

Our data may provide additional insight into DA, an endpoint that describes the orientation of a trabecular lattice, with more positive values representing a more highly oriented trabecular lattice [35]. Unlike SMI, DA was not only weakly correlated to BV/TV (SMI r values of ∼0.97 compared with mean r values of ∼0.32), as reported previously [39, 43], but also did not consistently differ from controls in the treatment groups that had higher bone strength (Tables 2, 3, 4, and 5), as previously noted [47]. However, DA was about 20 % higher in vertebral bodies than in the DFM, indicating that the vertebral body trabecular lattice is more strongly oriented than that of the distal femur. Others have found that regions such as the tibial epiphyseal and metaphyseal trabecular bone, in which the epiphysis has higher BV/TV than the metaphysis, do not always have higher DA [47]. The current data combine with others’ to suggest that DA describes fundamental properties of trabecular lattices from different anatomic regions [37], better than their BV or response to treatment.

We evaluated compressive strength of lumbar vertebral body specimens, in which maximum load ranged from 76–554 N. We also measured nondestructive descriptors of bone strength that included BV, Tb.N, Tb.Th, SMI, DA, cross-sectional area, and DBM. The endpoints themselves were highly correlated and therefore individually correlated to maximum load (Table 7). Tb.Th correlated best to maximum load (r = 0.81, Fig. 3). Cross-sectional area, an endpoint independent of trabecular microarchitecture, added additional information. Our results suggest that, in rats treated with anti-osteoporosis agents, measuring microarchitectural and bone size endpoints of the lumbar vertebral body appears to be a nondestructive surrogate endpoint for vertebral body compression strength. The principal microarchitectural change affected by PTH treatment in this and other studies of both rats and humans, is trabecular thickening [38, 48] (Fig. 3). This probably explains the primary association of bone strength with Tb.Th here, rather than with SMI [45], and shows that trabecular thickening by an anti-osteoporosis treatment agent is a reasonable microarchitectural property of trabecular bone that can be used to predict bone strength after therapy, particularly with PTH. The results also agree with human data that suggest that bone mass and bone size in the spine predict spine fracture risk [49].

Our preclinical study had several strengths. We studied a number of clinical treatment sequences of bone active agents, measuring both nondestructive surrogate measures of bone strength and bone strength itself. We used 90-day treatment periods, approximately two remodeling periods in mature adult rats, that each may represent up to 12–24 months in humans. We evaluated treatments, such as monotherapy with a bisphosphonate, Ral, and PTH, for which clinical fracture risk-reduction data exist. We measured surrogate bone strength endpoints in both the approved monotherapies, and other sequences of treatment for which clinical fracture risk data have not yet been collected, to enable predictions about which ones could offer improved fracture risk reduction compared with monotherapy.

However, there were also weaknesses. The dosing regimen of Ral at 5 mg/kg, 3×/week, while reported to prevent estrogen-deficient bone in previous studies [26, 31], was both a lower dose and less frequent dosing than what has been reported to produce the maximum possible effect of Ral on prevention of OVX-induced bone loss [20]. Moreover, we cannot extrapolate our preclinical study results to osteoporotic fracture risk in humans. Since we began treatment at 8 weeks post-OVX, a time when OVX-related bone loss was still ongoing, the findings may be best applied to women who are within the first few years of menopause.

In summary, we used an osteopenic adult OVX rat model to evaluate various sequential treatments for osteoporosis, using FDA-approved agents with complementary tissue-level mechanisms of action. Sequential treatment for 3 months each with a bisphosphonate, followed by an anabolic agent, followed by resumption of the bisphosphonate, created the highest trabecular bone mass, highest Tb.Th, highest Tb.N, lowest SMI, and highest bone strength. Any type of drug holiday, particularly a PTH holiday, resulted in loss of bone strength. These data, if confirmed in clinical studies, may assist clinicians in the long-term treatment of postmenopausal osteoporosis.

Acknowledgments

This work was funded by National Institutes of Health (NIH) grants R01 AR043052, 1K12HD05195801 and 5K24AR048841-09. Statistical support was made possible by grant no. UL1 RR024146 from the National Center for Research Resources (NCRR), a component of the NIH and NIH Roadmap for Medical Research. The involvement of ROR was supported by NIH (NIH/NIDCR) under grant no. 5R01 DE015633 to the Lawrence Berkeley National Laboratory (LBNL).

Conflicts of interest

None.

Copyright information

© International Osteoporosis Foundation and National Osteoporosis Foundation 2014

Authors and Affiliations

  • S. K. Amugongo
    • 1
  • W. Yao
    • 1
  • J. Jia
    • 1
  • Y.-A. E. Lay
    • 1
  • W. Dai
    • 1
  • L. Jiang
    • 1
  • D. Walsh
    • 1
  • C.-S. Li
    • 2
  • N. K. N. Dave
    • 3
  • D. Olivera
    • 3
  • B. Panganiban
    • 3
  • R. O. Ritchie
    • 3
  • N. E. Lane
    • 1
  1. 1.Center for Musculoskeletal Health and Department of MedicineUniversity of California Davis Medical CenterSacramentoUSA
  2. 2.Division of Biostatistics, Department of Public Health SciencesUniversity of CaliforniaDavisUSA
  3. 3.Lawrence Berkeley National Laboratory, and Department of Materials Science and EngineeringUniversity of CaliforniaBerkeleyUSA

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