Entry of DSIP peptides into dog CSF: role of physicochemical and pharmacokinetic parameters.

Delta sleep-inducing peptide (DSIP) or one of four DSIP analogs (desTrp-DSIP, D-Ala3-DSIP, D-Ala4-DSIP, D-Ala4-DSIP-NH2), was injected IV into dogs. Samples of cerebrospinal fluid (CSF) from the posterior fossa and blood were collected simultaneously at 2, 5, 10, 20, 30, 45, and 60 min after injection and concentrations of DSIP-like material measured by radioimmunoassay (RIA). The increase in peptide concentration in the CSF correlated significantly with the increase in plasma concentration, with plasma half-life, and with peptide lipophilicity, but not with the degree of plasma protein binding or with molecular weight. An even better correlation occurred when the three factors that correlated independently were combined into a single parameter (r = 0.813, p less than 0.00005). These results demonstrate that peptides can significantly penetrate the blood-CSF barrier of the dog. For DSIP and its analogs, the degree of penetration into the CSF is highly dependent on parameters such as the level of peptide in the plasma, plasma half-life, and the lipophilicity of the peptide.