Poly(ethylene carbonate)s, part II: degradation mechanisms and parenteral delivery of bioactive agents.

The degradation and drug carrier properties of poly(ethylene carbonate) (PEC) were investigated in vitro and in rats and rabbits. PEC was found to be specifically degraded in vivo and in vitro by superoxide radical anions O2-*, which are, in vivo, mostly produced by inflammatory cells. No degradation of PEC was observed in the presence of hydrolases, serum or blood. PEC is biodegraded by surface erosion without significant change in the molecular weight of the residual polymer mass. The non-hydrolytic biodegradation by cells producing O2-* is unique among the polymers used as biodegradable drug carriers. The main degradation product of PEC in aqueous systems is ethylene glycol, formed presumably by hydrolysis of ethylene carbonate. The splitting off of a five-membered ring structure from the polymer chain indicates a chain reaction mechanism for the biodegradation. PEC is a suitable drug carrier, particularly for labile drugs. Using human interleukin-3 and octreotide as model drugs, surface erosion of the PEC formulations was indicated by a 1:1 correlation between drug release and polymer mass loss.

Functional significance of interaction of H-FABP with sulfated and nonsulfated taurine-conjugated bile salts in rat liver.

To identify the cytosolic proteins of rat hepatocytes involved in transcellular transport of sulfated and taurine-conjugated bile salts in comparison with only taurine-conjugated bile salts, photoaffinity labeling studies were performed with [3H]-7,7-ASLCT (7,7-azi-3 alpha-sulfatolithocholyl[2'-3H(N)]taurine), [3H]-7,7-ACT ((7,7-azi-3 alpha,12 alpha-dihydroxy-5 beta-cholan-24-oyl)- 2'-[2'-3H(N)]aminoethanesulfonate), and [3H]-7,7-ALCT (7,7-azilithocholyl[2'-3H(N)]taurine) using isolated hepatocytes and intact liver tissue. Photoaffinity labeling of isolated hepatocytes with [3H]-7,7-ASLCT on the one hand and with [3H]-7,7-ACT and [3H]-7,7-ALCT on the other resulted in a different labeling pattern of cytosolic polypeptides, without a relevant incorporation of radioactivity into subunits of glutathione transferases. This suggests that glutathione transferases play no role in the transport of dianionic or of monoanionic bile salts. With [3H]-7,7-ACT and [3H]-7,7-ALCT a moderate incorporation of radioactivity was found in polypeptides with apparent M(r)s of 33,000, 38,000, and 54,000, whereas with [3H]-7,7-ASLCT, a polypeptide with an apparent M(r) of 14,000, identified as H-FABP, was markedly and almost exclusively labeled. Photoaffinity labeling of specimens of intact liver tissue resulted in a labeling pattern of cytosolic polypeptides comparable to that obtained from photolabeled isolated hepatocytes. All results suggest that transcellular transport of dianionic sulfated as well as taurine-conjugated bile salts and of monoanionic taurine-conjugated bile salts follows different pathways. In intracellular transport of taurine-conjugated bile salts, several cytosolic polypeptides may have a function, whereas, in transport of taurine-conjugated 3 alpha-sulfato bile salts, only H-FABP appears to be involved.

Synthesis of a metabolically stable modified long-chain fatty acid salt and its photolabile derivative.

An analogue of the long-chain fatty acid salt, sodium stearate, was synthesized in which the hydrogen atoms at carbons 2, 3, and 18 were replaced by fluorine. The key step in the synthesis was the addition of 3-iodo-2,2,3,3-tetrafluoropropanoic acid amide to 15,15,15-trifluoro-1-pentadecene. Radioactivity was introduced by catalytic reduction of 2,2,3,3,18,18,18-heptafluoro-4-octadecenoic acid amide with carrier-free tritium gas yielding a product with the specific radioactivity of 2.63 TBq/mmol. The resulting 2,2,3,3,18,18,18-heptafluoro-4-octadecenoic acid has a pKa of about 0.5 and is completely dissociated under normal physiological conditions. The fluorinated fatty acid salt analogue is readily taken up into hepatocytes and proved to be metabolically inert. In an approach to the identification of proteins involved in long-chain fatty acid salt transport across membranes and intracellular compartments, the photolabile derivative 11,11-azo-2,2,3,3,18,18,18-heptafluoro[G-3H]octadecanoic acid sodium salt was synthesized with a specific radioactivity of 2.63 TBq/mmol. Photolysis of the photolabile derivative, using a light source with a maximum emission at 350 nm, occurred with a half-life of 1.5 min. The generated carbene reacted with 14C-labeled methanol and acetonitrile with covalent bond formation of 6-13%. Its efficacy for photoaffinity labeling was demonstrated by incorporation into serum albumin, the extracellular fatty acid salt-binding protein, as well as into the intracellular fatty acid salt-binding protein (FABP) of rat liver with the molecular weight of 14,000.