Association constants of monoclonal antibodies for hapten: heterogeneity of frequency distribution and possible relationship with hapten molecular weight.

Using published data, we have investigated the relationship of the association constant (Ka) of 265 MAbs for haptens with molecular weights ranging from 111 to 1202 Da. The study indicates that: (1) differences of a factor 10(3)-10(5) are frequently found between the lowest and the highest value of Ka for the same hapten; (2) the relationship between log Ka and the hapten molecular weight of either the native drug or the molecular entity used for the Ka determination is described by a hyperbolic function; (3) beyond a critical molecular weight of approximately 300-325 Da, the log Ka reaches a plateau at a maximal value near 10(-12) M-1.