of B-amanitin which is then directly linked to amino groups 

 on proteins (Faulstich et al., 1975). This procedure should 

 not have the carbodiimide associated side reactions and 

 would lead to well defined, covalently linked conjugates. 

 To date, no additional work beyond the initial publication 

 has been presented to define the characteristics of these 

 conjugates. Furthermore, 3-amanitin is a relatively minor 

 component of the naturally occurring amatoxins and is not 

 readily available. 



Conjugates of the more widely available a-amanitin 

 were first prepared by the modification of a-amanitin to a 

 derivative containing a free amino group which can be 

 coupled to proteins via carbodiimides (Faulstich and 

 Trischmann, 1973). These derivatives when conjugated to 

 BSA were shown to be equally as toxic in vivo as free a- 

 amanitin. In vitro inhibition of calf thymus RNA polymerase 

 II by the a-amanitin-BSA conjugates was 20-fold less than 

 free a-amanitin. These studies as well as the previously 

 described investigations with g-amanitin conjugates pointed 

 out the feasibility of using amanitin-protein conjugates for 

 exploring the targeting of inhibitors to specific cells. 

 They also underlined the need for careful, quantitative 

 evaluation of the interaction of a well characterized con- 

 jugate with the cellular target. 



Investigations of drug targeting with inhibitors other 

 than amanitins have primarily centered on increasing the 

 specificity of uptake of an inhibitor by conjugation with 



