Healthy men between 18 and 55 years of age (inclusive), with body mass index within 18 to 32 kg/m2 (inclusive), were enrolled in both studies. Subjects were in good general health as determined by medical history, physical examination, vital signs, 12-lead electrocardiogram (ECG), and clinical laboratory measurements; they had negative test results for hepatitis B surface antigen, hepatitis C virus antibody, human immunodeficiency virus antibody, selected drugs of abuse, and breathalyzer alcohol and had not consumed nicotine or nicotine-containing products in 1 year before enrollment. All subjects used an acceptable method of contraception from day-1 until 30 days following the completion of studies.
Exclusion criteria were a significant history or clinical manifestation of any significant diseases or disorders, history of stomach or intestinal surgery or resection that would potentially alter absorption or excretion of drugs, abnormal ECG, or hypersensitivity reaction to the study drug or related compounds.
Subjects agreed to refrain from consuming grapefruit- or caffeine-containing products (for 72 h before day-1 until after collection of 96-h PK sample for the dose-escalation study and throughout the study for the dose-proportionality study) and alcohol-containing products (for 24 h before day-1 until the end of study in both studies). Additionally, they refrained from strenuous exercise from 48-h before day-1 and during the period of confinement at the clinical research unit (CRU) for the dose-escalation study and throughout the study for the dose-proportionality study.
Concomitant medications were not allowed during both studies with the exception of acetaminophen and non-steroidal anti-inflammatory drugs, which were allowed after the 96-h PK sample collection.
Both studies were conducted in accordance with the ethical principles that have their origin in the declaration of Helsinki and that are consistent with good clinical practices, applicable regulatory requirements, and in compliance with the respective protocols. The study protocols were reviewed and approved by an independent institutional review board (Florida, USA). All subjects provided written informed consent before entering the studies.
The dose-escalation study, conducted from December 17, 2008, to April 02, 2009, was an open-label study with a 14-day screening phase and up to 88-day treatment phase (approximately 22 days for each of the 4 single-dose cohorts). The dose-proportionality study, conducted from March 30, 2010, to May 1, 2010, was an open-label, randomized, four-way crossover study, with a 21-day screening phase and a 28-day treatment phase (consisting of 4 consecutive, 7-day treatment periods). Both studies were conducted at Covance Clinical Research Unit Inc., Texas, United States of America.
Abiraterone acetate (manufactured by Pharmaceutical International, Inc., Maryland) was provided as 250-mg tablets. In both studies, AA was administered at doses of 250, 500, 750, and 1,000 mg/day. In the dose-escalation study, these doses were administered to subjects in 4 dose cohorts in a sequential manner, starting with the lowest dose (250 mg/day). Dose administration for subsequent cohorts was started at least 15 days after dosing of the preceding cohort and after the safety data were reviewed and assessed as not having any safety concerns such as clinically significant adverse event indicative of drug intolerance; clinically significant finding for physical examination, 12-lead ECG, or vital signs; or clinically significant change in laboratory parameter. Due to an error in PK sample processing at the CRU for the Cohort 2 dose level (500 mg), another 500-mg dose level cohort was added and designated as Cohort 2a. The bio-analytical data for Cohort 2 was not evaluated.
In the dose-proportionality study, 32 eligible subjects were randomized (1:1:1:1) to receive 1 of the 4 treatment sequences: ADBC, BACD, CBDA, DCAB. Each treatment sequence included 4 doses of AA: 250 mg/day (treatment A), 500 mg/day (treatment B), 750 mg/day (treatment C), and 1,000 mg/day (treatment D). For each subject, dose administration for subsequent treatment in the treatment sequence was started 7 days after dosing of the preceding treatment.
All subjects fasted for at least 10 h before and 4 h after AA administration in both studies. Subjects were confined to CRU one day before every administration of AA and until collection of 24 h PK sample the following day. In addition, subjects visited CRU on days 3, 4, 5, 8, and 15 of each dose cohort in dose-escalation study and on days 3, 4, and 7 of each treatment period in the dose-proportionality study, for study procedures.
During both studies, blood samples were collected for the measurement of AA and abiraterone concentrations in plasma; samples were collected from the peripheral vein (via an indwelling catheter or direct venipuncture), within 0.5 h before the dose (predose sample) and at 0.25, 0.5, 1, 1.5, 2, 3, 4, 6, 8, 12, 24, 48, 72, and 96 h postdose. Additionally, in the dose-proportionality study, blood samples for evaluation of serum testosterone and LH were also collected on day 1 (predose and 2 and 12 h postdose), 2, 3, 4, and 5 of each cohort and at the end of study or early withdrawal visit.
Concentrations of abiraterone and AA were determined using a validated bioanalytical method employing ultra performance liquid chromatography with tandem mass spectrometric detection. The calibration range for abiraterone was 0.2–200 ng/mL for the dose-escalation study and 0.2–500 ng/mL for the dose-proportionality study. The calibration range for AA was 0.2–50 ng/mL for both studies.
For the dose-escalation study, non-compartmental analysis was performed using WinNonlin (Pharsight Corporation, Version 5.2). SAS version 9.1 was used for all statistical calculations, tables, and plots. For the dose-proportionality study, non-compartmental analysis was performed using WinNonlin (Version 4.0.1b). SAS version 9.1 was used for confidence interval testing. Tables and plots were created using PKAA 1.0, which utilized Microsoft Excel 2000.
Pharmacokinetic parameters calculated from plasma concentration–time data were maximum plasma concentration (C
max), time to reach C
max), and area under the plasma concentration–time curve (AUC) from 0 to the last measurable plasma concentration as calculated by the linear trapezoidal rule (AUC0–last); AUC extrapolated to infinity (AUC0–∞), calculated as: AUC0–∞ = AUC0–last + Ct/λz, where Ct was the last measurable plasma concentration and λz was the apparent terminal phase rate constant calculated as the magnitude of the slope of the linear regression of the log concentration versus time profile during the terminal phase; and apparent plasma terminal elimination half-life t
1/2 (whenever possible), where t
1/2 = (ln2)/λz.
Safety assessments included monitoring treatment-emergent adverse events (TEAEs), clinical laboratory tests, ECG, vital sign measurements, and physical examinations. The severity of adverse events (AEs) was assessed using the National Cancer Institute Common Terminology Criteria for AEs (NCI CTCAE, version 3.0, grades 1, 2, 3, and 4) . In addition, total testosterone and LH levels on days-1, 3, 5, 8, and 15 of each cohort in the dose-escalation study and on day 1 (predose and 2 and 12 h postdose) and day 2 through day 5 in each treatment period in the dose-proportionality study were evaluated. These evaluations were repeated approximately 7 days after the end of study, for subjects whose serum testosterone levels were not within the normal limits or below the baseline value at the end of study. In the dose-proportionality study, testosterone levels were monitored every 7 days until the levels returned to baseline or to within normal limits.
In both studies, PK analysis sets included all subjects who received the study drug and had at least 1 valid PK parameter estimate, and the safety analysis sets included all subjects who received at least one dose of study drug.
Plasma concentration data of abiraterone and all estimated PK parameters were summarized descriptively for both studies. For the dose-proportionality study, least squares means (LSM) and intra-subject variance were estimated using a mixed-effect model, which included treatment, period, and treatment sequence as fixed effects and participant as a random effect.
Sample size determination
Approximately 32 subjects were to be enrolled in the dose-escalation study based upon samples sizes used in previous PK studies of similar nature. For the dose-proportionality study, the sample size of 24 subjects (using an estimated intra-subject coefficient of variation of 21 % for AUC0–last and AUC0–∞) was sufficient for the point estimates of the ratio of AUC0–last and AUC0–∞ to fall within 90–111 % of the true value with 90 % confidence. This sample size was also sufficient for the point estimate of the ratio of C
max to fall within 86–116 % of the true value with 90 % confidence using an estimated intra-participant coefficient of variation of 31 % for C
max. Thirty-two subjects were randomized in the dose-proportionality study in order to obtain at least 24 subjects who completed the study.