Abstract C46: Design and identification of the novel hypoxia-activated irreversible pan-HER inhibitor SN29966
Authors: Smaill Jeff B. , Lu Guo-Liang , van Leeuwen Wouter , Abbattista Maria , Anderson Robert F. , Denny William A. , Doñate Fernando , Jaswail Jagdish , Maroz Andrej , Puryer Michelle , Syddall Sophie P. , Wilson William R. , Patterson Adam V. 
Published: June 2012

Abstract
Several irreversible pan-HER inhibitors (HKI-272, BIBW-2992, PF299) are under development and demonstrate encouraging activity against erlotinib resistant non-small cell lung cancer expressing mutant forms of HER1. However, dose limiting toxicities mirror that of erlotinib and are attributed to inhibition of HER1 in gastrointestinal and skin tissues. To introduce additional tumor selectivity to irreversible pan-HER inhibitors and thereby broaden their therapeutic index, we have sought to utilise tumor hypoxia as a physiological target that supports tumor-selective bioreduction. To achieve this we have developed a series of hypoxia-activated prodrugs of the known irreversible pan-HER inhibitor SN29926.
Five nitromethylaryl quaternary ammonium bromide (NMQ) prodrugs were synthesised by quaternizing the tertiary amine of SN29926 with five nitroheterocyclic alpha-methyl bromides. Three further quaternary salts were prepared as Chemical-Biology Tools (CBTs) designed as controls to probe aspects of the mechanism of action of the NMQ prodrugs. The NMQ prodrugs and CBTs were compared, relative to SN29926, in a range of assays to identify a lead compound that (i) is deactivated under oxic conditions with respect to A431 cellular HER1 target modulation and proliferation, (ii) efficiently fragments following one-electron reduction in an oxygen inhibited manner to release the irreversible pan-HER inhibitor, (iii) displays increased anti-proliferative activity against A431 cells under hypoxia.
All of the NMQ/CBT prodrugs were successfully deactivated with respect to inhibition of A431 HER1 autophosphorylation (62- to 201-fold) and proliferation under oxic conditions (12- to 294-fold). Pulse and steady-state radiolysis under nitrogen determined their one-electron reduction potentials (−388 to −493 mV) and demonstrated that only two of the NMQ prodrugs (SN29965, SN29966) efficiently fragment following one-electron reduction (first-order rate constants of 90 and 130 s−1, respectively). SN29966 alone demonstrated significant anti-proliferative activity against A431 cells under hypoxia (hypoxic/oxic IC50 ratio 38). Preliminary growth delay screening of SN29966 and the CBTs in advanced A431 xenografts (∼600 mm3; hypoxic fraction of 32% ± 13%) demonstrated that the CBTs lacked efficacy, while SN29966 induced tumor regressions, with greater than 30 days tumor control using a well tolerated three-dose schedule (133 umol/kg/dose, q4dx3). In summary, prodrug SN29966 is deactivated relative to the parent inhibitor, efficiently fragments following one-electron reduction, is selective against hypoxic A431 cells and has remarkable single-agent activity against hypoxic A431 xenografts using a conservative treatment schedule. We therefore identify SN29966 as a “first-in-class” hypoxia-activated irreversible pan-HER inhibitor that has significant clinical potential.
Citation Information: Mol Cancer Ther 2009;8(12 Suppl):C46.