Targeted delivery of a peripheral benzodiazepine receptor ligand-gemcitabine conjugate to brain tumors in a xenograft model

Abstract.

Purpose: Peripheral benzodiazepine receptors (PBRs) are overexpressed in brain tumors compared to normal brain, and could serve as a target to selectively increase anticancer drug delivery through a PBR ligand-drug conjugate system. We have previously synthesized PBR ligand-gemcitabine conjugates based on the model PBR ligand, PK11195. The goal of the current study was to examine this new drug delivery strategy in an intracerebral xenograft model by measurement of steady-state drug distribution following administration of gemcitabine (GEM) and PK11195-GEM. Methods: In vitro PBR receptor binding and cytotoxicity assays were used to screen three different PK11195-GEM conjugates (GG01, GG02, GG03) in human SF126 glioma cells. Based on these findings and the favorable chemical stability of GG01, here referred as PK11195-GEM, pharmacokinetic investigations of PK11195-GEM and GEM were conducted in male rats. These studies consisted of single-dose and steady-state dosing regimen studies, the latter to assess drug distribution in normal brain and brain tumors. PK11195-GEM and GEM were measured in blood and tissue samples by HPLC. Results: All PBR-GEM conjugates demonstrated appreciable receptor binding affinity and cytotoxicity with mean IC₅₀ values ranging from 248 to 376 nM and 5.6 to 29.1 nM, respectively. The cytotoxicity of GEM was comparable with a mean IC₅₀ value of 5.9 nM. Following administration of single 8 mg/kg doses of PK11195-GEM to rats (n=4), PK11195-GEM had a mean total clearance of 126.3±29.6 ml/min per kg, and a volume of distribution at steady-state of 1261.9±31.05 ml/kg that resulted in a very short elimination half-life of 16.1±5.8 min. In comparison GEM had a similar volume of distribution (993.8±131.6 ml/kg), reduced clearance (3.4±0.8 ml/min per kg), and longer half-life (235.6±26.7 min). In nude rats bearing intracerebral tumors, mean steady-state tumor/plasma, tumor/right brain, and tumor/left brain PK11195-GEM concentration ratios were 1.75±0.46, 5.49±5.2, and 9.96 ±3.2, respectively. The analogous values following GEM administrations were 0.81±0.5, 3.67±1.57, and 5.21±1.95, respectively. These values indicate a minimum twofold increase in tumor target selectivity for the conjugate delivery system compared to GEM treatment. Conclusion: Targeting intracellular PBRs is a new drug delivery strategy based on the use of low molecular weight drug conjugates that can be administered systemically. It was demonstrated under steady-state conditions that PK11195-GEM possessed a twofold enhancement in brain tumor selectivity compared to GEM alone. This type of target selectivity would allow higher tumor concentrations to be achieved in conjunction with lower drug concentrations in normal or non-target tissues.