Abstract B76: Cellular metabolism, murine pharmacokinetics and preclinical antitumor activity of SN29966, a novel hypoxia-activated irreversible pan-HER inhibitor

Abstract

Abstract Hypoxia occurs in most human tumors and is associated with disease progression, resistance to conventional therapies and poor patient outcome. Hypoxia can up-regulate HER1 by several known mechanisms, including increased mRNA translation (Franovic et al., PNAS,2007;104:13092) and delayed receptor endocytosis (Wang et al., Nat Med.,2009;15:319). We have developed SN29966, a hypoxia-activated prodrug of the irreversible pan-HER inhibitor SN29926, to target and exploit hypoxic cells and thereby broaden the therapeutic index of this class of agent. Quaternization of the aminebearing Michael acceptor masks activity SN29926, and one-electron reduction provides selective release under hypoxia. This prodrug design confers 62-fold deactivation of inhibitor activity with respect to HER1 autophosphorylation in A431 cells (Smaill et al., this meeting). In a panel of HER1/2 expressing cell lines SN29966 showed hypoxia-dependent inhibition of proliferation (hypoxic/oxic IC50 ratios of 63, 38, 31 and 19 in BT474, A431, SKOV3 and SKBR3 cells, respectively), a property SN29926 lacked (IC50 ratios 0.7–1.1). SN29926 was generated from SN29966 under hypoxic conditions at a rate of 200–500 pmol/hr/106 cells. Oxic production was ∼1 pmol/hr/106 cells. Plasma and A431 tumor pharmacokinetics (PK) of prodrug SN29966 and inhibitor were measured in nude mice by LC/MS/MS detection (with D6 internal standards) following administration at their respective MTDs (133 and 75 umol/kg; ip). Prodrug SN29966 gave a plasma AUC0-72h of 2016 umol-h/L, some ∼110-fold greater than achieved for administration of inhibitor SN29926 (18 umol-h/L). The latter gave a tumor AUC0-inf of 100 umol-h/kg with a half-life (t½) of 9 h. In contrast the prodrug SN29966 gave a tumor AUC0-72h of 2245 umol-h/kg with a stable tumor tissue concentration of ∼ 30 umol/kg out to 72 h, such that a t½ could not be determined. Consistent with this long prodrug residency, SN29926 released from prodrug had a t½ in tumor tissue of >72h, providing an AUC0-72h of 464 umol-h/kg. Thus the AUC of SN29926 in A431 tumors was at least 4.6-fold higher after administration of prodrug SN29966 than following administration of inhibitor SN29926 itself at equivalent toxicity. In A431 tumor growth delay studies, SN29966 (113 umol/kg, q4dx6, ip) induced tumor regressions (12/12) with no recovery of growth by day 36 (tumor volume 44 ±19 mm3), whereas controls grew rapidly (day 8 tumor volume 1043 ±109 mm3). Comparative administration of inhibitor SN29926 (63 umol/kg, q4dx6, ip) provided tumor stasis, but with rapid growth following cessation of treatment (day 36 tumor volume 642 ±191 mm3), a difference that was significant from prodrug (p<0.01). On this dose and schedule toxicity parameters were comparable, with body weight loss at nadir (day 23) of 5.4% ±1.3% (0/12 deaths) for SN29966 versus 9.8% ±1.3% (2/12 deaths) for SN29926. Superior SN29966 activity was also seen in the SKOV3 xenograft model and was associated with suppression of HER2 phosphorylation. Collectively the data indicate a marked therapeutic advantage for prodrug SN29966 over inhibitor SN29926 in both A431 and SKOV3 tumor models. The PK parameters of SN29966 and the durable release of inhibitor in tumors may explain the robust antitumor activity of this new class of agent. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B76.