A sequential cohort study evaluating single-agent KappaMab and KappaMab combined with lenalidomide and low-dose dexamethasone in relapsed and/or refractory kappa light chain-restricted multiple myeloma (AMaRC 01-16).

KappaMab (KM; formerly MDX-1097) is a monoclonal antibody specific for the kappa myeloma antigen (KMA), a cell-surface antigen expressed on malignant plasma cells in kappa-restricted multiple myeloma (κMM), some lymphomas, occasional tonsillar B cells and in vitro activated B cells, but not on normal B cells in bone marrow. Phase I/IIa studies of single-agent KM confirmed a favourable toxicity profile and evidence of anti-myeloma activity. Ex-vivo studies demonstrating upregulation of KMA by lenalidomide, and enhanced effector-cell cytotoxicity provided the rationale for this phase IIb study where KM or KM in combination with lenalidomide and dexamethasone (KM-Rd) was administered in relapsed, refractory κMM patients. In addition, outcomes for a real-world matched case-control cohort from the Australian and New Zealand Myeloma and Related Diseases Registry (MRDR) who received Rd were compared to the KM-Rd cohort. KM-Rd demonstrated an overall response rate of 82.5% which compared favourably to the Rd-MRDR cohort of 45.1%. Both single-agent KM and KM-Rd regimens were well tolerated, with the KM-Rd safety profile similar to patients given only Rd in other clinical settings. Based on the excellent safety profile and significant efficacy, further clinical trials escalating the KM dose and pairing KM with other standard-of-care treatments are planned.

MDX-1097 induces antibody-dependent cellular cytotoxicity against kappa multiple myeloma cells and its activity is augmented by lenalidomide.

MDX-1097 is an antibody specific for a unique B cell antigen called kappa myeloma antigen (KMA) that consists of cell membrane-associated free kappa light chain (κFLC). KMA was detected on kappa human multiple myeloma cell lines (κHMCLs), on plasma cells (PCs) from kappa multiple myeloma (κMM) patients and on κPC dyscrasia tissue cryosections. In primary κMM samples, KMA was present on CD38+ cells that were CD138 and CD45 positive and/or negative. MDX-1097 exhibited a higher affinity for KMA compared to κFLC and the latter did not abrogate binding to KMA. MDX-1097-mediated antibody-dependent cellular cytotoxicity (ADCC) and in vitro exposure of target cells to the immunomodulatory drug lenalidomide resulted in increased KMA expression and ADCC. Also, in vitro exposure of peripheral blood mononuclear cells (PBMCs) to lenalidomide enhanced MDX-1097-mediated ADCC. PBMCs obtained from myeloma patients after lenalidomide therapy elicited significantly higher levels of MDX-1097-mediated ADCC than cells obtained prior to lenalidomide treatment. These data establish KMA as a relevant cell surface antigen on MM cells that can be targeted by MDX-1097. The ADCC-inducing capacity of MDX-1097 and its potentiation by lenalidomide provide a powerful rationale for clinical evaluation of MDX-1097 alone and in combination with lenalidomide.

Soluble expression of a functional recombinant cytolytic immunotoxin in insect cells.

We have previously described the production of a recombinant melittin-based cytolytic immunotoxin (IT), scFv-mel-FLAG, in bacterial cells. While the IT exhibited specific cytotoxicity for a human lymphoblastoid cell line, HMy2, yields from expression were low. Here, we describe a baculovirus expression system for the overexpression and secretion of scFv-mel-FLAG. A novel snake phospholipase A2 inhibitor signal peptide was used to aid in the secretion of the immunotoxin. Sf21 insect cells infected with the recombinant virus secreted soluble scFv-mel-FLAG into the culture medium from which it was purified directly on an affinity column. The final yield of scFv-mel-FLAG was estimated at 3-5 mg/L, which was an improvement of 30-fold compared to expression in the prokaryotic system. The cell binding characteristics of the recombinant IT were assessed by flow cytometry using the antigen expressing cell line HMy2. ScFv-mel-FLAG bound specifically to HMy2 cells in direct binding assays and this binding was completely inhibited in the presence of an excess of soluble antigen. Significant cytotoxicity for HMy2 cells, measured by leakage of cytosolic LDH, was also observed for the IT at a concentration of 60 pmol/10(4) cells. Cytotoxicity was concentration dependent and was specific for antigen-positive cells. Thus the baculovirus expression system, under the control of a novel secretion signal, can be used for the production of soluble and functional recombinant cytolytic immunotoxins. To our knowledge, this is the first report of expression of a recombinant immunotoxin in the baculovirus expression vector system.