A New Antibody-Drug Conjugate Composed of an Anti-HLA-DR IgG4 Antibody, IMMU-114, and SN-38, Is Active in Experimental Acute Myelocytic Leukemia (AML), Acute Lymphocytic Leukemia (ALL), and Multiple Myeloma (MM)

Introduction: Relapsed AML, ALL, and MM continue to be a therapy challenge. IMMU-114 (hL243) is a humanized anti-HLA-DR IgG4 monoclonal antibody engineered to lack effector-cell functions, but retains binding and a broad range of antitumor effects in diverse hematological neoplasms (Stein et al., Blood. 2010;115:5180-90). When given subcutaneously, it has encouraging efficacy in an initial Phase I clinical trial in relapsed or refractory NHL and CLL, with a good safety profile (ClinicalTrials.gov, NCT01728207).

In vitro, AML has proven to be resistant to the antitumor effects of IMMU-114, despite high expression levels of HLA-DR. Likewise, in several different human ALL and MM cell lines, IMMU-114 has demonstrated a range of antitumor effects from a low of 9% to a high of 69%. In an effort to improve the antitumor activity of IMMU-114, an antibody-drug conjugate (ADC) was made in which IMMU-114 was conjugated with the active metabolite of irinotecan, SN-38. Another ADC utilizing SN-38 (sacituzumab govitecan) being studied in solid tumors has been well tolerated, with clinically significant objective responses in patients given multiple cycles over >6 months, with manageable neutropenia being the major toxicity. Thus, our goal was to determine if SN-38, a drug not commonly used in hematopoietic cancers, would prove to be an effective and safe therapeutic when targeted with the IMMU-114 antibody.

In this current work, the in vivo activity of hL243-SN-38 versus parental IMMU-114 is examined in human AML, ALL, and MM xenografts.

Methods : Conjugation of SN-38 to hL243 IgG4 resulted in a drug-to-antibody-ratio range of 6.1 to 6.6. For AML and MM disease models, NSG/SCID and C.B.-17 SCID mice received 2 Gy irradiation 24 h prior to an i.v. injection of MOLM-14 (2x10⁶) or CAG cells (1x10⁷), respectively. ALL was established in C.B-17 SCID mice injected with MN-60 cells (1x10⁷). All therapies began 5 days post-tumor-cell injection. Test agents, including a non-targeting anti-CEA-SN-38 ADC, were administered as 500-mg injections twice-weekly for 4 wks. Animals were sacrificed at disease progression, characterized by the onset of hind-limb paralysis or loss of more than 15% body weight.

Results: In experimental AML, saline control and IMMU-114 treated mice succumbed to disease progression quickly, with a median survival time (MST) of only 14 and 15 days, respectively. Conversely, mice treated with hL243-SN-38 had a greater than 1.5-fold increase in survival (MST=37 d, P=0.0031). Further, hL243-SN-38 therapy provided a significant survival benefit when compared to anti-CEA-SN-38 control (MST=21 d, P=0.0031). In mice bearing ALL xenografts, IMMU-114 provided a >60% improvement in survival compared to saline control (MST = 37 d vs. 22.5 d, respectively; P<0.0001), whereas hL243-SN-38 increased this by another 80% (MST=66.5 d) which was significantly better than all other treatments, including IMMU-114 (P<0.0001). Finally, MM mice had a greater than 123-d MST when treated with hL243-SN-38 compared to 32 d for saline control (P<0.0001). This survival benefit also was significantly higher than for mice treated with bortezomib (0.89 mg/kg) or control ADC (MST=32.5 d for both; P<0.0001). While not significant, hL243-SN-38 does provide a >30% improvement in survival when compared to IMMU-114 therapy (MST=94.5 d, P=0.1313). In all three experiments, therapy with hL243-SN-38 was well tolerated, as evidenced by no significant loss in body weight.

Conclusions: Therapy with the hL243-SN-38 ADC proved to be superior to IMMU-114 (which is active clinically in NHL and CLL) in both AML and ALL xenografts and beneficial in MM. Most importantly, in IMMU-114-refractive AML, hL243-SN-38 demonstrated a significant antitumor effect without any undue toxicity. This new ADC is a candidate for clinical evaluation in these intractable malignancies.