Clinical data indicate that mutant EGFR NSCLC is often heterozygous (PLoS ONE 2013; 8: e54170; PLoS ONE 2009; 4: e7464) and the presence of wild type (WT) EGFR allele is associated with limited response to EGFR-tyrosine kinase inhibitor (TKI) therapy (Cancer Sci 2008; 99:929). Tumor hypoxia upregulates WT EGFR protein and its cognate ligand TGFα via several HIF-dependent mechanisms (reviewed in: Curr Pharm Des 2013; 19:907). NSCLC is known to be a hypoxic tumor, and thus hyperactivation of WT EGFR may be an important cause of resistance to EGFR-TKI therapy. TH-4000 (formerly called PR610) is a clinical-stage hypoxia-activated prodrug that releases an irreversible EGFR-TKI under hypoxic conditions and may overcome resistance to conventional TKI therapy. We tested this hypothesis using the heterozygous WT/Δ19 EGFR PC9 tumor model and found it to be resistant to clinically relevant doses of the EGFR-TKI erlotinib; 100% of tumors progressed during treatment with human matched plasma PK exposures of erlotinib. In contrast, the homozygous Δ19 mutant EGFR tumor HCC827 was readily controlled by erlotinib (100% tumor regression). TH-4000 (15 mg/kg) produced 100% tumor regressions in both models. In vitro, PC9 cells exposed to hypoxia had elevated EGFR protein and were more resistant to erlotinib as measured by EGFR phosphorylation. In nude mice, single-dose administration of 15 mg/kg TH-4000 achieved a plasma AUC equivalent to 32 mg/m2 in human subjects, one-fifth of the maximum tolerated dose (MTD) defined in the Phase 1 trial (MTD = 150 mg/m2/week; NCT01631279). A single dose of TH-4000 (15 mg/kg) cleared rapidly from mouse plasma (T½ = 0.37 h) but had durable residency in PC9 tumors (T½ = 39 h), releasing TKI above efficacious levels for 7 days (T½β = 84 h). Consistent with these PK properties, tumor shutdown of EGFR signalling was durable, with no recovery by day 7. To confirm the mechanism of action, TH-4000 was shown to be metabolized efficiently under hypoxia using a panel of human NSCLC cell lines (rate of TKI release 0.4-2.1 nmol/hr/106 cells), a process that was inhibited by oxygen (TKI release < 0.002 nmol/hr/106 cells). Cellular anti-proliferative and receptor phosphorylation assays demonstrated a 14-80 fold reduction of TH-4000 activity relative to TKI. Using PC9 tumors, hyperbaric oxygen breathing suppressed release of TKI from TH-4000 by >80% (538 vs 99 nmol/kg; p <0.01) compared to air breathing controls. Collectively, these data further validate that TH-4000 is a hypoxia-activated irreversible EGFR-TKI, and show that TH-4000 has greater activity compared with erlotinib in a heterozygous xenograft model of mutant EGFR NSCLC. Thus, TH-4000 may overcome hypoxia-induced resistance to erlotinib at plasma exposures readily achieved in human subjects.

Citation Format: Adam V. Patterson, Shevan Silva, Christopher Guise, Maria Abbattista, Matthew Bull, Huai-Ling Hsu, Charles Hart, Jessica Sun, Angus Grey, Amir Ashoorzadeh, Robert Anderson, Jeff B. Smaill. The hypoxia-activated EGFR-TKI TH-4000 overcomes erlotinib-resistance in preclinical NSCLC models at plasma levels achieved in a Phase 1 clinical trial. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 5358. doi:10.1158/1538-7445.AM2015-5358