Optimisation of alendronate conjugation to polyethylene glycol for functionalisation of biopolymers and nanoparticles

Abstract The coupling of alendronate (ALN) to heterobifunctional polyethylene glycol (PEG) requires an orthogonal synthetic route. This can be achieved using thiolation of alendronate with 2-iminothiolane and subsequent Michael addition to maleimide functionalised PEG. This paper evaluates the chemical reactions between 2-iminothiolane and alendronate and demonstrates how reaction conditions previously reported for thiolation of amines are not ideal in reactions where the amine has a high pKa value, such as for amine-containing bisphosphonates. Furthermore, the optimised method for thiolation of alendronate is applied to the synthesis of alendronate functionalized PEGs, ALN-PEG-NH2 and ALN-PEG-COOH, with isolation and full characterization of these two products. Further conjugation of the latter onto the biopolymer chitosan is demonstrated, including evaluating reagent ratios on the degree of substitution.