Albumin−Insulin Conjugate Releasing Insulin Slowly under Physiological Conditions:  A New Concept for Long-Acting Insulin

The covalent linkage of peptides or protein drugs to human serum albumin (HSA) greatly prolongs their lifetime in vivo but is pharmacologically irrelevant when it irreversibly inactivates them. We retain drug bioactivity by synthesizing a heterobifunctional reagent (MAL-Fmoc-OSu, 9-hydroxymethyl-2-(amino-3-maleimidopropionate)-fluorene-N-hydroxysuccinimide) that generates HSA−Fmoc−insulin on covalent conjugation to the amino group of insulin and the Cys-34 side chain of HSA. HSA−Fmoc−insulin is water-soluble and, upon incubation in aqueous buffers reflecting normal human serum conditions, slowly, spontaneously, and homogeneously hydrolyzes to release unmodified insulin with a t 1/2 of 25 ± 2 h. A single subcutaneous or intraperitoneal administration of HSA−Fmoc−insulin to diabetic rodents lowers circulating glucose levels for about 4 times longer than an equipotent dose of Zn2+-free insulin. Following subcutaneous administration, onset of the glucose-lowering effect is delayed 0.5−1 h and persists for 12 h. Thus, we present a prototype insulin formulation possessing three desirable parameters: high aqueous solubility, delayed action following subcutaneous administration, and prolonged therapeutic effect.