Abstract A247: Mechanism of action of the hypoxia-activated irreversible pan-HER inhibitor SN29966.
Authors: Smaill Jeff B. , Jaiswal Jagdish , Abbattista Maria , Lu Guo-Liang , Anderson Robert F. , Ashoorzadeh Amir , Denny William A. , Donate Fernando , Hsu Huai-Ling , Lee Ho H. , Maroz Andrej , Mehta Sunali , Pruijn Alison , Puryer Michelle , Syddall Sophie P. , Thompson Aaron , van Leeuwen Wouter , Wilson William R. , Jamieson Stephen , Patterson Adam V. 
Published: June 2012

Abstract
Hypoxia occurs in most human tumors and is associated with disease progression, treatment resistance and poor patient outcome. We have developed the hypoxia-activated prodrug SN29966, designed to release the irreversible pan-HER inhibitor SN29926, following one-electron reduction by hypoxic cells (Smaill et al, Mol Cancer Ther., 2009; 8(12 Suppl), C46). Pharmacokinetic (PK) studies in nude mice bearing A431 tumor xenografts indicated SN29966 has a long tumor half-life (&gt;3 days) and releases SN29926 in tumors. SN29966 demonstrated single agent activity in nude mice bearing A431 and SKOV3 xenografts, inducing striking tumor regressions in both models (Patterson et al, Mol Cancer Ther., 2009; 8(12 Suppl), B76). PR509 and PR610, clinical candidates developed from SN29966, are currently undergoing comparative evaluation with Phase I trials anticipated in early 2012.
The single-agent antitumor activity of SN29966 is arguably counter-intuitive given that it is designed to target hypoxic cells within tumors. This activity may arise from a number of contributing mechanisms including; (i) bioactivity of the unreduced prodrug; (ii) local redistribution of released inhibitor in the tumor; (iii) liver metabolism and circulating inhibitor and (iv) a long tumor half-life allowing for targeting of both chronic and cycling hypoxia. To critically assess the relative contribution of each to the mechanism of action of SN29966 we performed a number of studies. We prepared SN31950, a prodrug of SN29926 designed to be incapable of one-electron fragmentation. In target modulation and anti-proliferative assays SN31950 showed no hypoxia-dependent activity. The murine A431 tumor PK of SN29966 and SN31950 demonstrated that at an equimolar dose (20 μmol/kg, ip), both prodrugs gave comparable tumor exposures (AUC0–72h: SN31950, 50 μmol*h/kg; SN29966, 57 μmol*h/kg). In contrast, the tumor exposure of SN29926 released from each prodrug differed by 40-fold (AUC0–72h: SN29926 from SN31950, 0.3 μmol*h/kg; SN29926 from SN29966, 12 μmol*h/kg). Plasma exposure of each prodrug was comparable, as were levels of SN29926 in plasma (presumed mainly due to hepatic prodrug metabolism). Consistent with the observed lack of inhibitor release in A431 tumors, SN31950 was inactive against A431 tumors in growth delay assays. To confirm the hypoxia-dependent nature of SN29966 inhibitor release in A431 tumors we re-oxygenated tumors in mice breathing 100% oxygen at 2.5 atm in a hyperbaric chamber. Accordingly, mice showed a marked reduction (56%, p&lt;0.001) in the concentration of released inhibitor in tumor relative to air breathing controls, indicating SN29966 tumor metabolism is hypoxia-dependent. Conversely, post-mortem anoxia (2 h, 37 °C) in mice 24 hours after SN29966 administration doubled the concentration of released inhibitor in tumor, indicating SN29966 is bioavailable for metabolism to release inhibitor as de novo hypoxia arises. Collectively the data support the conclusion that the single-agent activity of SN29966 results primarily from hypoxia-dependent tumor metabolism and inhibitor release. Inhibitor bystander effect and exploitation of both chronic and cycling hypoxia, thereby increasing the target cell population, are likely to play important roles. Liver metabolism, circulating inhibitor and activity of unreduced prodrug appear to be less important.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A247.