Enhanced stability in vitro and in vivo of immunoconjugates prepared with 5-methyl-2-iminothiolane

Substituted 2-iminothiolanes (X2ITs) are new heterobifunctional crosslinking agents designed for the preparation of disulfide-linked conjugates with enhanced resistance to reduction. Based upon 2-IT substituted at the 4 and/or 5 position, these reagents appear to function by sterically protecting the conjugate disulfide bond from attack by thiolate nucleophiles. Here, we have used the X2ITs to prepare and evaluate a series of immunoconjugates (antibody-cytotoxin conjugates) between the murine monoclonal antibody 791/T36, which recognizes a 72-kDa surface antigen present on many human tumor cells, and RTA30, the naturally occurring 30-kDa glycoform of ricin A chain. The X2IT-linked conjugates were also compared to immunoconjugates prepared with N-succinimidyl 3-(2-pyridyldithio)-propionate (SPDP) and 4-[(succinimidyloxy)carbonyl]-alpha-methyl-alpha-(2- pyridyldithio)toluene (SMPT), as well as with methyl- and dimethyl-substituted structural analogs of SPDP. In vitro, 791-(X2IT)-TNB model compounds exhibited a 6000-fold range of stabilities. In contrast, the corresponding 791-(X2IT)-RTA30 immunoconjugates were up to 20-fold more stable than conjugates made with unhindered linkages. These improvements resulted in immunoconjugates with prolonged serum half-lives in animals. Our data indicate that one of the crosslinking agents, 5-methyl-2-iminothiolane (M2IT), has optimal properties for the preparation of disulfide crosslinked immunoconjugates intended for therapeutic use in that (i) it is highly water soluble and reacts rapidly with protein amino groups at neutral pH, preserving the positive charge, (ii) it forms conjugates with RTA30 efficiently, and (iii) its conjugates exhibited enhanced disulfide bond stability in vitro and in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)