Immunoconjugates are being explored as novel
cancer therapies with the promise of target-specific
drug delivery. The
immunoconjugate,
huN901-DM1, composed of the humanized monoclonal
IgG1 antibody, huN901, and the maytansinoid
drug, DM1, is being tested in clinical trials to treat
small cell lung carcinoma (SCLC).
huN901-DM1 contains an average of three to four DM1
drug molecules per huN901 antibody molecule. The
drug molecules are linked to huN901 through random modification of huN901 at epsilon-amino groups of
lysine residues, thus yielding a heterogeneous population of conjugate species. We studied the
drug distribution profile of
huN901-DM1 by electrospray time-of-flight mass spectrometry(ESI-TOFMS), which showed that one to six DM1
drug molecules were attached to an antibody molecule. Both light and heavy chains contained linked drugs. The conjugation sites in both chains were determined by
peptide mapping using
trypsin and Asp-N
protease digestion.
Trypsin digestion identified modified
lysine residues, since these residues were no longer susceptible to enzymatic cleavage after conjugation with the
drug. With respect to Asp-N digestion, modified
peptides were identified by observing a mass increase corresponding to the modification. The two digestion methods provided consistent results, leading to the identification of 20 modified
lysine residues in both light and heavy chains. Each
lysine residue was only partially modified. No conjugation sites were found in
complementarity determining regions (CDRs). Using structural models of human
IgG1, it was found that modified
lysine residues were on the surface in areas of structural flexibility and had large
solvent accessibility.