Starch microspheres as carriers for X-ray imaging contrast agents: Synthesis and stability of new amino-acid linker derivatives

Abstract

The relative stability of particulate contrast agents for X-ray imaging, consisting of a succinic-acid derivative of a water-soluble X-ray contrast agent bound to starch particles through an amino-acid ester bond, has been studied. To investigate the effect of chain length of the amino-acid linker on degradation rate, two glycine-linked derivatives and a corresponding β-alanine-linked derivative were prepared as model compounds. The cleavage rate of the amino-acid ester bond in the starch particle β-alanine derivative had a significantly lower cleavage rate than in the corresponding glycine derivatives; after 22 h in human blood serum at 37 °C the remaining fraction of the undegraded β-alanine-linked derivative was 78%, while 31.1 and 29.3% were the remaining fractions of the two glycine-linked derivatives. The cleavage data correlated well with biphasic cleavage processes with two distinct half lives for the respective pseudo first order processes. The second preparation with the glycine linker had a cleavage profile and rate equivalent to that of the first one in human blood serum, but the corresponding hydrolysis in phosphate buffer was significantly slower with 79.9%-remaining fraction after 22 h, and was apparently a monophasic pseudo first order reaction. Variation of the degradability of the starch matrix had apparently no significant effect on the cleavage rates of the linker. This suggests that components in the human blood serum catalyze the cleavage of the ester bond in these derivatives and that a fraction of the covalently bound contrast agent had a significantly slower cleavage rate from the matrix. The derivative with a glycine linker between the carrier matrix and the contrast agent is a promising candidate for liver and spleen directed X-ray contrast with respect to density of contrast generating iodine and biodegradability.