Marked Therapeutic Efficacy of a Novel Polyethyleneglycol-SN38 Conjugate, EZN-2208, in Xenograft Models of Non-Hodgkin’s Lymphoma.

Background: The clinical utility of SN38 (10-hydroxy-7-ethyl-camptothecin), the active moiety of CPT-11 (Camptosar®), has been severely compromised due to its poor solubility. EZN-2208 is a novel, water-soluble, polyethyleneglycol (PEG)-SN38 conjugate generated by linking SN38 with a multi-arm high molecular weight PEG, 40k 4-arm-PEG, via a glycine linker. Here, we compare the efficacy of EZN-2208 and CPT-11 in the treatment of animals bearing non-Hodgkin’s lymphomas.

Methods: The in vitro cytotoxicity of EZN-2208 and CPT-11 were tested in human lymphoma (Raji, Daudi, DoHH2) cancer cell lines using the tetrazolium assay. The pharmacokinetics and maximum tolerated dose (MTD) of EZN-2208 and CPT-11 were evaluated in tumor-free severe combined immunodeficient (SCID) mice and the therapeutic efficacy was evaluated in xenograft models of non-Hodgkin’s lymphoma (Raji and Daudi).

Results: In vitro, the IC₅₀ of EZN-2208 ranged from 2–20nM and the in vitro potency of EZN-2208 was 30 to 50-fold more than CPT-11. The MTDs of EZN-2208 and CPT-11 in SCID mice were 30 and 60 mg/kg, respectively, when injected as a single dose. When administered in a multiple dose regimen (q2d x 5), the MTDs of EZN-2208 and CPT-11 were 10- and 40 mg/kg respectively. The pharmacokinetic profile of EZN-2208 was biphasic showing a rapid distribution phase (t_1/2a =0.6h) and a slow elimination phase (t_1/2b =19.3h for conjugate and 14.2h for SN38). In the Raji xenograft model, treatment with a single MTD of EZN-2208 resulted in a 500% improvement in life span (ILS) and 50% cures of animals compared with 19% ILS observed for mice treated with a single MTD of CPT-11. Multiple dose treatment of EZN-2208 resulted in 90% cures of animals compared with 63% ILS and no cures observed for the CPT-11 group. In the Daudi xenograft model, a single injection of EZN-2208 given as early treatment resulted in cures of 100% of animals in contrast to 66% ILS and no cures observed for CPT-11 group. When treatment was delayed, a single MTD of EZN-2208 caused 90% cures of animals, whereas CPT-11 treatment was completely ineffective.

Conclusions: EZN-2208 demonstrated excellent efficacy in preclinical models of non-Hodgkin’s lymphoma. Ongoing Phase I studies will determine the optimal dose and schedule of EZN-2208.