A second-generation DNA-binding pyrrole-imidazole polyamide with antitumor activity.

Abstract

e13588 Background: Pyrrole-imidazole polyamides are synthetic minor groove binders with programmable sequence recognition. A polyamide that targets the DNA sequence 5’-WGWWCW-3’ (W=A/T) found in the Androgen Receptor (AR) response element inhibits a subset of AR driven transcripts and is cytotoxic in a number of cancer cell lines including LNCaP (prostate cancer), without evidence of DNA damage. Pharmacokinetic studies show bioavailability after intravenous or subcutaneous injection. This polyamide suppressed growth of LNCaP tumor xenografts in mice. However, escalated dosing resulted in significant weight loss that could deter further development. We now report a second-generation polyamide with decreased systemic toxicity that retains efficacy towards LNCaP xenografts. Methods: We investigated the toxicity of our lead polyamide in normal (C57BL/6)mice by necropsy. We then synthesized a focused library of four polyamides that retain the DNA-recognition sequence of our lead molecule, but with chemical substitutions at non-recognition elements. We treated groups of normal male mice (n=4 each) with escalating doses up to 10 mg/kg and observed them for 10 days or until signs of significant distress, after which mice were necropsied. The least toxic polyamide was then tested for suppression of LNCaP xenograft growth (n=14 per treatment arm: 1 mg/kg polyamide versus saline vehicle alone). Tumors were allowed to grow to 200 mm3 prior to treatment by subcutaneous injection every three days for a cycle of six injections. Tumors were then resected and measured. Results: The primary target organ of toxicity for our lead polyamide was the liver, which demonstrated toxicity at 3 mg/kg and dose-limiting toxicity at less than 10 mg/kg. One polyamide in our library demonstrated no evidence of toxicity by necropsy and serum analysis up to 10 mg/kg. This polyamide also suppressed growth of established LNCaP xenografts (T/C: 51%, p=0.0073) at a dose of 1 mg/kg. The other polyamides demonstrated greater toxicity and were not tested further. Conclusions: We report a second-generation polyamide with minimal systemic toxicity in rodents that retains in vivo antitumor activity in prostate cancer xenografts.