Abstract 2544: Preclinical pharmacodynamics (PD) of ONX 0801, a folate receptor-α (FRα) and tumor-targeted thymidylate synthase (TS) inhibitor
Authors: Ng Chau HM , Valenti Melanie , Ruddle Ruth , Raynaud Florence , Theti Davinder S. , Mitchell Fraser , Jackman Ann L. 
Published: June 2012

Abstract
ONX 0801, a first-in-class tumor-targeted antifolate, selectively targets TS in tumor cells due to transport by FRα, thus differentiating it from other antifolates. FRα expression is restricted to the apical membrane of specialised tissues generally inaccessible to drugs in circulation; however, it is highly expressed in several cancers including ovarian. FRα is a high affinity, low capacity transporter and the rate of drug uptake and cell line sensitivity to ONX 0801 is related to the number of FRαs tethered to the cell surface (72h IC50 range: 0.003-0.3µM). Non-FRα mediated uptake becomes relevant at relatively high concentrations (&gt;1µM). The purpose of the study was to investigate the relationship between drug exposure and FRα- or non-FRα mediated TS inhibition in both tumors and normal proliferating tissues. PD endpoints of TS inhibition included an intact TS assay for cultured cells, and tumor and plasma dUrd in mice bearing FRα positive IGROV-1 ovarian tumors. In IGROV-1 cells, the 72h IC50 was 0.003µM; however, blocking FRα-mediated uptake with excess folic acid increased it to 2µM. In FRα negative A431 cells the IC50 was 7µM. Drug washout experiments demonstrated that exposure to high concentrations for &lt; 24h gave a duration of TS inhibition insufficient to produce cytotoxicity as ONX 0801 is not polyglutamated and therefore effluxed from non-FR expressing cells. The IC50 for ONX 0801 in A431 cells was &gt;30µM for 4h exposure (72h endpoint) but was reduced to 8µM and 5µM for a 24h and 48h exposure, respectively. These data suggest that this level of exposure in vivo could produce effects in normal proliferating tissues (NPT). We have tested this in mice bearing IGROV-1 xenografts by injecting ONX 0801, as either a bolus to achieve temporary high exposure, or as a 3 day continuous infusion (CI) with osmotic minipumps. During the 4hr following a bolus of 100-200mg/kg of ONX 0801 the plasma drug levels were high (∼500-1µM) and plasma dUrd was elevated consistent with non FR-mediated TS inhibition in NPT. However, plasma dUrd had normalised by 24h. Consistent with tumor-targeting, tumor dUrd was still elevated 24h after 15mg/kg ONX 0801. CI of ONX 0801 of 5mg/kg/24h produced a steady-state plasma drug level of 0.05µM and increased tumour dUrd for 72h. No increases in plasma dUrd were observed up to a dose of 300mg/kg/24h where a small increase was seen (plasma ONX 0801 ∼4µM). Taken together, these data are consistent with cell line data and predict that increasing exposure in patients to &gt;5µM for &gt;24h is unnecessary and could be associated with toxicity. In conclusion, ONX 0801 has a high therapeutic index in model systems using PD markers of TS inhibition. Further, we have defined guiding pharmacokinetic and PD parameters for the ongoing Phase 1 clinical trial to aid the determination of the biologically effective dose. Supported by grants from Onyx Pharmaceuticals, BTG PLC and Cancer Research UK
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2544. doi:10.1158/1538-7445.AM2011-2544