Abstract
Antibody-drug conjugates (ADCs), potent cytotoxic drugs linked to antibodies via specialized chemical linkers, provide a means to increase the effectiveness of chemotherapy by targeting the drug to neoplastic cells while reducing side effects. Our research has focused on ADCs that contain the maytansinoid DM1, monomethylauristatin E (MMAE), or monomethylauristatin F (MMAF), all of which are potent inhibitors of microtubule polymerization. These drugs are attached to the antibody via several different linker formats (Table 1).
Table 1 Linker-drug Linker Mechanism Linked via Drug Cell Permeable PAB-vc-MC-MMAE Cleavage of peptide bond Cysteine Yes MC-MMAF Uncleavable Cysteine No SPP-DM1 Reduction of disulfide bond Lysine Yes MCC-DM1 Uncleavable Lysine Yes
We identified seven cell surface proteins expressed in non-Hodgkin lymphoma (NHL) with limited expression patterns in normal tissue (CD19, CD20, CD21, CD22, CD72, CD79, and CD180), as potential therapeutic targets for ADCs. ADCs directed to any of these seven targets are effective in xenograft models when containing linkers that can be cleaved in the tumor microenvironment or when internalized by tumor cells. However, ADCs with uncleavable linkers, i.e., linkers that are active when antibody is internalized and degraded within cells, are effective only when targeted to CD22 or CD79b. This suggests that these antigens provide the desired intracellular targeting of the cytotoxic drug. Further, we demonstrated that the uncleavable-linker anti-CD22 and anti-CD79b ADCs result in complete tumor regression with no recurrence in some xenograft models of NHL. In vitro experiments revealed that sensitivity of a given cell line to the ADCs anti-CD22-MCC-DM1 and anti-CD79b-MCC-DM1, correlated more with sensitivity of the cell line to the corresponding free drug, than to the amount of surface expression of the target or the amount of internalized ADC. We conducted pilot safety studies in rats (non-binding species) and cynomolgus monkeys (binding species). In rats, ADCs with uncleavable linkers caused less target-independent hepatic and hematologic toxicity than their counter parts with cleavable linkers, likely due to their decreased systemic release of free drug. Because of these results, we evaluated three ADCs with uncleavable linkers (anti-CD22-MCC-DM1, anti-cynomolgus monkey CD79b-MCC-DM1, and anti-CD22-MC-MMAF) at ∼30 mg/kg q3 weeks for 2 doses in cynomolgus monkeys. Normal B-cells were depleted in circulation and in tissue, particularly in germinal centers of lymphoid tissues; an expected pharmacological effect. Other findings included clinically tolerated and reversible elevations in liver enzymes (MCC-DM1 and MC-MMAF conjugates) and decreases in platelet counts (MCC-DM1 conjugates); and minimal to mild sciatic nerve degeneration (MCC-DM1 conjugates). The data demonstrate that anti-CD22 and anti-CD79b ADCs with uncleavable linkers have potent efficacy in xenograft models of NHL, favorable safety profiles in pilot animal studies, and suggest that for specific targets, uncleavable linkers provide a promising mechanism to improve margins of safety in humans.
Eradicating mesothelin-positive human gastric and pancreatic tumors in xenograft models with optimized anti-mesothelin antibody–drug conjugates from synthetic antibody libraries
