Treatment of acute lymphoblastic leukemia with CMC-544 (inotuzumab ozogamicin), a CD22-targeted immunoconjugate of calicheamicin

915 Inotuzumab ozogamicin (CMC-544) is a CD22-specific cytotoxic immunoconjugate of a derivative of calicheamicin (CalichDMH) that has demonstrated significant anti-tumor activity in a phase I clinical trial in heavily pre-treated B-NHL patients (Blood 2005;106:230a). Calicheamicins are potent cytotoxic enediyne antibiotics that bind DNA in the minor groove and cause double strand DNA breaks (dsDNAB) leading to cell death. In this preclinical study, CMC-544 was investigated as an anti-tumor agent against human B-cell Acute Lymphoblastic Leukemia (B-ALL). The majority of patients with B-ALL express CD22 on the surface of the leukemic cells. The expression of CD22 on the surface of three distinct human B-ALL cell lines used in this study was lower than that on a Ramos B-cell lymphoma (BCL) line. However, the growth inhibitory effect of CMC-544 against B-ALL cells was more potent than that against Ramos BCL. dsDNABs in malignant B-cells caused by CMC-544 or unconjugated CalichDMH were monitored as 53BP1 foci. dsDNABs caused by short-term (1h) exposure to CalichDMH were more pronounced in B-ALL cells than in Ramos BCL suggesting that B-ALL cells were more sensitive to the DNA-damaging effects of calicheamicin. As a reflection of CD22-targeted delivery of calicheamicin, CD22-targeted CMC-544 produced a much greater amount of dsDNABs in CD22+ CD33- B-ALL cells than CD33-targeted gemtuzumab ozogamicin. When evaluated in vivo, CMC-544 (administered ip) caused regression of established Reh B-ALL xenografts in nude mice. A dose of CMC-544 as low as 10 μg of conjugated CalichDMH/kg was able to significantly inhibit growth of Reh B-ALL xenografts whereas long-term cures were achieved with a dose of 160 μg/kg of CMC-544. A mixture of the targeting humanized antibody in CMC-544, G5/44, and unconjugated CalichDMH had no significant effect on the growth of Reh B-ALL xenografts. When human CD22+ CD33- CD45+ Reh B-ALL cells were injected iv into scid mice and allowed to disseminate systemically, mice developed hind-limb paralysis induced by infiltration of the disseminated B-ALL cells into the central nervous system. Treatment of scid mice with disseminated B-ALL with CMC-544 completely prevented the hind limb paralysis and ensuing death. Mice with disseminated B-ALL cells that were treated with CMC-544 showed a dramatic reduction in the number of human CD45+ cells engrafted in the bone marrow. These results demonstrate that B-ALL cells are more sensitive than their BCL counterparts to the cytotoxic effects of CalichDMH and that the targeted delivery of CalichDMH via CMC-544 may prove to be an effective treatment for CD22+ B-ALL.