Leucovorin rescue allows effective high-dose pralatrexate treatment and an increase in therapeutic index in mesothelioma xenografts

Abstract

Purpose: To investigate the ability of leucovorin (LV) to abrogate dose-limiting toxicities of pralatrexate (PDX) while maintaining efficacy, in vivo.

Methods: H2052 mesothelioma cells were treated with the antifolates methotrexate (MTX), PDX and pemetrexed, with and without LV rescue 24 h later. Cell killing was evaluated 48 h later. Female nude mice bearing H2052 xenografts were treated with varying doses and schedules of the antifolate PDX and LV.

Results: In vitro, H2052 cells were more sensitive to PDX as compared to MTX and pemetrexed. Administration of LV 24 h after antifolate treatment reduced efficacy of antifolates MTX and pemetrexed, but not PDX. In vivo, LV was found to reduce toxicity of PDX at the maximum tolerated dose without sacrificing efficacy. Lethal doses of PDX were rescued by LV, and mice bearing the H2052 tumor demonstrated prolonged and enhanced tumor regression.

Conclusions: High-dose PDX with subsequent LV rescue may be a viable treatment strategy in mesothelioma and other cancers. The inclusion of LV rescue into new and existing PDX treatment protocols should be explored as a way to expand the tolerability and effectiveness of PDX in the clinic.

Fig. 1

Leucovorin (LV) may abrogate pralatrexate (PDX) activity through three mechanisms. i Competition for reduced folate carrier type 1 (RFC) transport into cell. ii Competition for polyglutamylation, a retention and activity marker, by folylpolyglutamate synthase (FPGS). iii Provides an alternate source of tetrahydrofolate, working around PDX inhibition of dihydrofolate reductase (DHFR)

Fig. 2

Pralatrexate (PDX) is more potent than methotrexate and pemetrexed in vitro. Dose response curves shown as percent of viable H2052 mesothelioma cells remaining after 24 h treatment with pralatrexate (a), methotrexate (b) or pemetrexed (c) followed by 72 h culture in fresh media (solid line) or followed by 24 h culture in 2μM leucovorin and 48 h culture in media (dashed line). d A summary table of IC50 values

Fig. 3

A and B Pralatrexate is effective against H2052 xenografts. Mice bearing 100 mm³ tumors were treated by i.p injection of saline control; 60 or 180 mg/kg PDX on days 1, 4 and 7; 60 or 180 mg/kg PDX on days 1, 4 and 7 followed by 50 mg/kg leucovorin 24, 32 and 48 h after each PDX administration; 50 mg/kg leucovorin. Tumor volume (a) and body weight (b) were measured