Preparation and pharmacokinetics of labeled derivatives of apamin

Apamin, the specific, centrally acting peptide neurotoxin from bee venom, was radiolabeled by different methods in order to study its in vitro and in vivo fate.

Iodination of the histidyl residue resulted in a derivative of diminished toxicity whose label was partially lost within 1 week. Furthermore, it was adsorbed on to Sephadex G-25 gel in a non-specific manner.

Acetylation with ³H-acetic anhydride yielded a slightly less toxic and, according to its behaviour on carboxymethyl cellulose, less basic derivative as compared with the starting material. Acetyl apamin was degraded in vitro by homogenates from liver, kidney, and brain, but not by serum, yielding ³H-acetic acid. In vivo it was degraded to tritiated water.

Reductive alkylation with formaldehyde and sodium borohydride resulted in methylated apamin possessing the basicity and about half the toxicity of the starting peptide. It was also degraded in vitro and in vivo, although to a lesser degree than acetyl apamin. Reductive alkylation appears to be the most favourable approach towards biologically active, labeled apamin.

The radiological or metabolic instability of the different apamin derivatives prevented a quantitative pharmacokinetic approach towards the in vivo fate of intact apamin. However, it is already evident that its acetyl or N-methyl derivatives are enriched not in the pharmacodynamic target organ, i.e. the central nervous system, but in the kidney.