Farletuzumab, a monoclonal antibody against folate receptor alpha (FRα), does not block folate or anti-folate binding or alter drug potency assessed in vitro.

Abstract

e13544 Background: FRα is overexpressed in a number of carcinomas. It is the target of the humanized monoclonal antibody, farletuzumab (FAR), currently in trials for ovarian and NSCL cancer. Pemetrexed (PMX) is commonly used in NSCLC treatment. Because PMX has a high affinity for FRα, it would be of concern if its accumulation depended on FRα and FAR blocked this process. Prior work using siRNA technology has shown that the cytotoxicity of PMX in FRα + cell lines is not dependent on receptor (Clin Ca Res 2004; 10:7986-93). Methods: Here we assessed the membrane binding, cellular accumulation and the cytotoxicity of PMX as well as methotrexate (MTX) and aminopterin (AMT), anti-folates with a very low affinity for FRα, in cell lines that express FRα [IGROV (ovarian carcinoma), MA104 (kidney epithelial)] or FRα negative Chinese Hamster Ovary (CHO) cells. In addition, since patients are supplemented with folate while taking PMX, we also assessed effects of FAR on the binding and uptake of the predominant plasma folate, 5-methyltetrahydrofolate (CH3FH4). Results: Binding of radiolabeled PMX and CH3FH4 was not blocked by FAR even when pre-incubated for 72 h prior to drug addition. Moreover, when cells were incubated with folic acid (FA), a ligand with such a high affinity for FRα that any cell accumulation of test drug is via a receptor independent mechanism (e.g., reduced folate carrier), there was significant CH3FH4 and PMX uptake, indicating that FRα is not the sole path for transport. FAR did not alter the IC50 of PMX, MTX or AMT, but increasing CH3FH4 from 5 to 40 nM increased the IC50 of PMX from 1.5 to 20 nM. FA (100 nM) increased the IC50 of PMX to 28 nM. Conclusions: Whereas folate decreases the potency of PMX, FAR does not interfere with folate or anti-folate binding by FRα and, moreover, does not alter the potency of anti-folates in vitro. This implies that FAR and small molecules targeting FRα may be used concomitantly if preclinical testing warranted such combinations and that the addition of FAR to a regimen already using PMX would not be expected to result in marked increases in PMX toxicity.