PURPOSE: L612, a human IgM monoclonal antibody produced by an EBV-transformed 
human B-cell line, binds to ganglioside GM3 and kills GM3-positive human 
melanoma cells in the presence of complement. It has been shown to be effective 
in some patients with late-stage melanoma. L612 consists of hexameric IgM (about 
20%), pentameric IgM (about 74%), and other minor IgM molecules. Because 
hexameric IgM activates complement more effectively than pentameric IgM, we 
developed and evaluated a hexamer-dominant recombinant IgM for clinical 
applications.
EXPERIMENTAL DESIGN: Chinese hamster ovary (CHO) cells were transfected with 
heavy- and light-chain genes of L612, with or without the joining-chain gene. 
Antitumor effects of the recombinant IgM secreted from CHO cells were evaluated 
in vitro and in vivo.
RESULTS: Recombinant IgM secreted from CHO cells without the joining chain 
(designated CA19) was approximately 80% hexameric, whereas recombinant IgM from 
CHO cells transfected with heavy-, light-, and joining-chain genes (designated 
CJ45) was about 90% pentameric. Both CA19 and CJ45 recombinant IgMs caused 
complement-dependent cytotoxicity against human and mouse melanoma cell lines, 
but the amount of CA19 required for 50% specific cytotoxicity was 5 to 10 times 
smaller. I.v. injection of CA19 compared with CJ45 or native L612 elicited more 
profound antitumor activity in nude rats bearing a GM3-positive mouse melanoma 
xenograft.
CONCLUSIONS: A hexamer-dominant human IgM against GM3 may provide a more potent 
treatment option for patients with GM3-positive melanoma.