SYNTHESIS OF COBALAMIN DIMERS USING ISOPHTHALATE CROSS-LINKING OF CORRIN RING CARBOXYLATES AND EVALUATION OF THEIR BINDING TO TRANSCOBALAMIN-II

Several cobalamin (Cbl) dimers have been prepared for evaluation as po tential antiproliferative agents in the treatment of AIDS-related lymp homa. The Cbl dimers were synthesized by cross-linking Cbl carboxylate s, produced by acid hydrolysis of the b-, d-, and e-propionamide side chains of cyanocobalamin (CN-Cbl), through an isophthalate molecule. L inking molecules were used between the Cbl carboxylates and the isopht halate moiety. The linkers were incorporated to provide a distance bet ween the two Cbl molecules such that the dimeric Cbls might bind two m olecules of transcobalamin II (TCII), the Cbl transport protein in pla sma. Initially, the linking moiety used was 1,12-diaminododecane, but the resulting dimers had low aqueous solubility. To improve the solubi lity of the dimers, 4,7,10-trioxa-1,13-tridecanediamine was employed a s the linking moiety. This improved the water solubility of the dimers considerably, while retaining the distance between the Cbl molecules at 41-42 Angstrom (fully extended). To introduce additional substituti on on Cbl dimers, 5-aminoisophthalic acid was used as the cross-linkin g reagent. p-Iodobenzoyl and p-(tri-n-butylstannyl)benzoyl conjugates of 5-aminoisophthalate were synthesized and used to prepare Cbl dimers . The stannylbenzoyl-conjugated Cbl dimers were prepared as precursors to be used in radioiodination reactions, and the iodobenzoyl-conjugat ed Cbl dimers were prepared as HPLC standards for the radioiodinated p roduct. Attempts to iodinate/radioiodinate the stannylbenzoyl Cbl dime rs were unsuccessful. Although an explanation for this is not readily apparent, the failure to react may be due to the lipophilicity of the linker used and the steric environment of the two Cbl moieties. A biot inylated derivative of 5-aminoisophthalate was also synthesized and us ed to prepare biotinylated-Cbl dimers. In a competitive rhTCII binding assay with [Co-57]CN-Cbl, Cbl dimers containing the lipophilic diamin ododecane linking moiety had decreased binding avidities compared to t hose of Cbl monomers substituted at the same corrin ring carboxylate. However, Cbl dimers containing the water-solubilizing trioxadiamine li nker appeared to have avidities similar to those of the Cbl monomers.