New modular delivery system for diagnostic and therapeutic pre‐targeting using tautomer‐specific monoclonal antibody EM‐6‐47 and 3‐substituted adenines

We have developed a new modular affinity system for the 2‐step delivery of functional molecules to target cells. The system is based on the tautomer‐specific monoclonal antibody (MAb) EM‐6–47, which binds to 3‐ and 3,8‐substituted adenines with high affinity (K a > 10 9 l/mol) without cross‐reacting with naturally occurring purine derivatives. This MAb serves as the hapten‐specific fusion partner to produce bispecific MAbs (bs‐MAbs) recognizing a target cell antigen and a low‐m.w. hapten as carrier molecule for, e.g. , radionuclides. Either the C‐8 or the N‐3 position of adenines can be used for conjugation with effector molecules; the remaining position may be substituted with different moieties to modulate the pharmacokinetics of the haptens. Different 3‐ and 3,8‐substituted adenines conjugated to the chelates DOTA and DTPA or to the drug daunomycin were synthesized. Adenine‐chelate derivatives were efficiently labeled with 111 In and 90 Y, while high‐affinity binding of 3‐substituted adenines to MAb EM‐6–47 remained almost unaffected by the conjugation to radiochelates. To confirm the validity of the delivery system, a prototype bs‐MAb, EM‐168–47, was generated by somatic cell fusion of MAb EM‐6–47 and MAb EM‐168–2, the latter recognizing a surface antigen on canine hematopoietic cells. Two‐step targeting assays in vitro verified the bs‐MAb‐mediated, dose‐dependent delivery of 111 In‐labeled adenine‐chelate derivatives to myeloid cells. This system represents a powerful tool for new pre‐targeting approaches relying on bs‐MAbs and low‐m.w. haptens. Suitable cellular antigens can be targeted by fusing the appropriate MAbs with hapten‐specific MAb EM‐6–47, and tailor‐made 3‐substituted adenines may be labeled with diagnostic or therapeutic radionuclides, cytotoxic drugs or other functional molecules. Int. J. Cancer 77:610–619, 1998. © 1998 Wiley‐Liss, Inc.