Targeting CD74 with Novel Antibody Drug Conjugates (ADCs) for the Treatment of B-Cell Non-Hodgkin's Lymphoma (NHL)

CD74 is a type II transmembrane glycoprotein involved in the formation and transport of MHC class II protein. High expression of CD74 has been confirmed in follicular lymphoma, diffuse large B-cell lymphoma (DLBCL), chronic lymphocytic leukemia (CLL)/small lymphocytic lymphoma (SLL) and other types of NHL with immunohistochemistry (IHC) using the LL1 antibody (Stein et al. Clin Cancer Res 2007). We employed site-specific conjugation technology to generate novel CD74-targeting ADCs, SP7676 and SP7675 (STRO-001) that exhibit a high degree of homogeneity characterized by the drug linker covalently binding to a single defined site. The human anti-CD74 IgG1 antibody (SP7219) used for ADCs SP7676 and STRO-001 was engineered using novel Fab-based ribosome display methods enabling selection from ~10¹² different antibody variants. Hundreds of unique Fabs from this selection were converted to IgGs and expressed directly in Sutro's proprietary cell-free protein synthesis platform, Xpress CF^TM, for extensive screening. The top antibody lead derived from this screen is further tested to identify the best sites for conjugation of linker-warheads. Sutro's SP7219 emerged as the top performing antibody and was conjugated to noncleavable DBCO-maytansinoid linker-warheads to form the ADCs SP7676 and STRO-001. Since conjugation sites were selected based on highest stability both in vitro and in vivo, these novel ADCs lose little drug moiety in circulation and have potential for improved PK, safety and activity profiles. In vitro cell proliferation/cytotoxicity assays show potent activity in 1) DLBCL (germinal center B-cell-like [GCB] and "double-hit") lines: SU-DHL-4, IC₅₀ - 1nM; SU-DHL-6, IC₅₀ - 0.4 nM; WSU-NHL, IC₅₀ - 1.6 nM; Pfeiffer, IC₅₀ - .09 nM; NUDUL-1, IC₅₀ - 0.4 nM; HT, IC₅₀ - 0.7 nM; OCI-LY-19, IC₅₀ - 0.7 nM; WSU-DLBCL2, IC₅₀ - 0.3 nM; 2) mantle cell lymphoma (MCL) cell lines: Mino, IC₅₀ - 0.4-0.7 nM; JVM-2, IC₅₀ - 1.7-2.9 nM; Jeko-1, IC₅₀ - 0.4 - 0.6 nM; 3) Ph+ acute lymphoblastic leukemia (ALL): SUP-B15, IC₅₀ - 3.9-4.6 nM; and 4) CLL (EBV-transformed): JVM-13, IC₅₀ - 3.0-3.4 nM. SP7676 elicited strong anti-tumor response in the OCI-LY-10 lymphoma xenograft model with 100% of animals achieving complete regression of tumors at 3mg/kg every 3 days x 5 doses and 10 mg/kg weekly x 3 doses. In the WSU-DLCL2 "double-hit" lymphoma xenograft model, administration of SP7676 (with re-dosing at time of re-growth) produced tumor regressions at 10 mg/kg every 3 days x 5 (6/8 mice tumor free, remaining 2 with small tumors) and 10 mg/kg weekly x 3 (tumor regression in most animals, 4/8 tumor free). Additionally, STRO-001 exhibits dose-dependent tumor growth inhibition in SUDHL-6 xenografts starting at 2.5 mg/kg weekly x 3 doses. Exploratory testing of our lead candidate, STRO-001 in cynomologous monkeys showed dose-dependent B-cell depletion at 1 - 30 mg/kg doses on Day 1 and 15, confirming the intended pharmacodynamic effect. Our preliminary data demonstrate that SP7676 and STRO-001 generate potent cell killing activity across multiple B-cell lymphoma/leukemia cell lines in vitro, and anti-tumor activity in preclinical B-cell NHL xenografts. Evaluation of STRO-001 in other cell lines and xenograft models and in combination studies is ongoing. GLP toxicology and other IND-enabling studies are planned.