Immunoassay of racemic drugs: a problem of enantioselective antisera and a solution.

Although it has been demonstrated that immunization with drug-protein conjugates derived from enantiomerically pure drug derivatives leads to stereoselective antisera, the effect of the use of conjugates of racemic drugs on antibody stereoselectivity has received little attention. Antisera were developed by immunizing rabbits with a conjugate (1) of the hemisuccinate of the antimalarial drug 1-(1,3-dichloro-6-trifluoromethyl-9-phenanthryl)-3-N, N-dibutylaminopropan-1-ol (WR 171,669) and bovine thyroglobulin. These antisera in binding competition studies with tritium-labeled racemic WR 171,669 demonstrated different stereoselective for d-, l- and dl-WR 171-669. Immunization with a conjugate (II) made from a nonchiral analog of the drug led to antisera which showed little discrimination between the optical isomers. Hydrolysis of conjugate I showed no excess of one enantiomer over the other, thus indicating that the stereoselectivity was the result of stereoslective processing of the conjugate by the immune system. The use of antisera from racemic conjugates to analyze total drug levels must be examined in each case ot avoid errors. The use of conjugate achiral drug analogs leading to the production of antisera relatively "blind" to enantiomeric differences is one solution to the problem of selective antisera.