Epratuzumab–SN-38: A New Antibody–Drug Conjugate for the Therapy of Hematologic Malignancies

Abstract

We previously found that slowly internalizing antibodies conjugated with SN-38 could be used successfully when prepared with a linker that allows approximately 50% of the IgG-bound SN-38 to dissociate in serum every 24 hours. In this study, the efficacy of SN-38 conjugates prepared with epratuzumab (rapidly internalizing) and veltuzumab (slowly internalizing), humanized anti-CD22 and anti-CD20 IgG, respectively, was examined for the treatment of B-cell malignancies. Both antibody-drug conjugates had similar nanomolar activity against a variety of human lymphoma/leukemia cell lines, but slow release of SN-38 compromised potency discrimination in vitro even against an irrelevant conjugate. When SN-38 was stably linked to the anti-CD22 conjugate, its potency was reduced 40-to 55-fold. Therefore, further studies were conducted only with the less stable, slowly dissociating linker. In vivo, similar antitumor activity was found between CD22 and CD20 antibody-drug conjugate in mice-bearing Ramos xenografts, even though Ramos expressed 15-fold more CD20 than CD22, suggesting that the internalization of the epratuzumab-SN-38 conjugate (Emab-SN-38) enhanced its activity. Emab-SN-38 was more efficacious than a nonbinding, irrelevant IgG-SN-38 conjugate in vivo, eliminating a majority of well-established Ramos xenografts at nontoxic doses. In vitro and in vivo studies showed that Emab-SN-38 could be combined with unconjugated veltuzumab for a more effective treatment. Thus, Emab-SN-38 is active in lymphoma and leukemia at doses well below toxic levels and therefore represents a new promising agent with therapeutic potential alone or combined with anti-CD20 antibody therapy.

Milatuzumab–SN-38 Conjugates for the Treatment of CD74+ Cancers

“…3C), consistent with other CL2A-SN-38 conjugates (30,31). The CL2E-SN-38 conjugate, however, was highly inert, with a half-life extrapolated to approximately 87.5 days.…”

“…Milatuzumab (30,31), as were the milatuzumab-doxorubicin conjugates (13,14). All conjugates were prepared by disulfide reduction of the IgG, followed by reaction with the corresponding maleimide derivatives of these linkers (Fig.…”

“…The CL2A-linked SN-38 conjugate is an exception, as more than 90% of the SN-38 is released from the conjugate into the media over the 4-day assay period (30,31). Thus, even if this conjugate was internalized rapidly, it would be difficult to discern differences between the free drug and the CL2A-linked drug.…”

“…Previous in vivo studies with the milatuzumab-doxorubicin or -SN-38 conjugates prepared with various antibodies had indicated they were efficacious at doses far lower than their maximum-tolerated dose (13,14,26,30,31), and thus in vivo testing focused on comparing similar, but fixed, amounts of each conjugate at levels that were tolerated. For example, weight loss never exceeded 10% of the starting weight, except in the intravenous Raji model, where weight loss was often indicative of progressive disease, not toxicity.…”

“…Prior studies indicated a preference for a linker (CL2A) that allowed SN-38 to dissociate from the conjugate in serum with a half-life of approximately 1 day, rather than other linkers that were either more or less stable in serum. However, given milatuzumab's exceptional internalization capability, a new linker that is highly stable in serum, but can release SN-38 when taken into the lysosome, was developed (31).…”