Steady-state pharmacokinetic properties of pramipexole in healthy volunteers.

Pramipexole is a dopamine receptor agonist that has proved effective in the treatment of Parkinson's disease. The pharmacokinetic properties of pramipexole at steady-state concentrations were studied in 16 healthy men and women at four dose levels throughout the range recommended for Parkinson's patients. Plasma and urine samples collected within the four dose intervals were assayed for concentrations of pramipexole, using high-performance liquid chromatography. The total oral clearance for all participants was 419 mL/min. The mean volume of distribution and elimination half-life for all participants was 486 +/- 93.2 L and 12.9 +/- 3.27 hours. Concentrations of pramipexole were proportional to dose, although the drug's pharmacokinetic properties differed between men and women. The area under the concentration-time curve for each dose level was 35% to 43% greater in women, mainly because of a 24% to 27% lower oral clearance. The mean creatinine clearance in men and women was 112 +/- 12.8 mL/ min/1.73 m2 and 80.9 +/- 15.6 mL/min/1.73 m2, respectively. The renal clearance of pramipexole accounts for approximately 80% of oral clearance, and there was a significant correlation between renal and creatinine clearances. The influence of gender could not be distinguished from the influence of age and the resulting reduced creatinine clearance, but the measurement of pharmacokinetic properties produced linear results in both men and women.

Structure-activity relationships of KNEFIRFamide (AF1), a nematode FMRFamide-related peptide, on Ascaris suum muscle.

Analogues of KNEFIRFamide (Lys-Asn-Glu-Phe-Ile-Arg-Phe-NH2; AF1), an FMRFamide-related peptide (FaRP) originally isolated from Ascaris suum, were characterized in an A. suum muscle tension assay. AF1 had biphasic effects on this preparation, inducing a brief relaxation followed by excitation and spastic paralysis. Activity of AF1 in this assay was eliminated by N-terminal deletions and by deamidation of the carboxy-terminus. The potency of AF1 was greatly reduced by alanine substitution for any residue. Peptides that retained activity did not show the biphasic response observed with AF1, suggesting that the inhibitory and excitatory phases seen with AF1 may be due to activation of distinct receptors. The basis for the marked differences in potency observed between AF1 and the structurally related nematode FaRP, AF2 (KHEYLRFamide) was also tested. AF2 is approximately 1000-fold more potent than AF1 in this assay, but has physiological effects that are otherwise indistinguishable. KNEYIRFamide and KNEFLRFamide induced characteristic AF1/AF2 responses, but were much less potent than the native peptides. In contrast, KHEYIRFamide resembled AF1 in potency and pattern of responses. These data suggest that AF1 and AF2 act at distinct receptors, and hypothesis supported by the observation that KNEFIAFamide antagonized the effects of AF1 but not of AF2.

Position 2 and position 2/Ala15-substituted analogs of bovine growth hormone-releasing factor (bGRF) with enhanced metabolic stability and improved in vivo bioactivity.

In order to prepare GRF analogs with high activity in vivo, a strategy was undertaken to stabilize the peptide to dipeptidylpeptidase IV (DPP-IV), a protease found in plasma which inactivates native human and bovine GRF by cleavage of the Ala2-Asp3 bond. Replacement of the Ala2 residue with Ser, Thr, or Gly in [Leu27]bGRF(1-29)NH2 resulted in peptides greatly stabilized against proteolysis in plasma, but having low inherent GH-releasing activity when tested in bovine pituitary cell cultures. Replacement of Gly15 with Ala15 was marginally effective in improving the in vitro bioactivity of this group of peptides. When tested for GH-hormone release in steers, however, the Thr2,Ala15 analog was four times more potent than bGRF(1-44)NH2. Eleven additional analogs from the [X2,Ala15,Leu27]bGRF(1-29)NH2 series were synthesized and evaluated for metabolic stability in bovine plasma and for GH releasing activity in steers in vivo and in rat pituitary cells in vitro. Two compounds, [Val2,Ala15,Leu27]dGRF(1-29)NH2 and [Ile2,Ala15,Leu27]-bGRF(1-29)NH2, had increased GH-releasing activity in steers over that of [Thr2,Ala15,Leu27]-bGRF(1-29)NH2 and over a previously reported super-potent analog, [desNH2Tyr1,D-Ala2,Ala15]-hGRF(1-29)NH2.

Growth hormone-releasing factor analogs with hydrophobic residues at position 19. Effects on growth hormone releasing activity in vitro and in vivo, stability in blood plasma in vitro, and secondary structure.

To test the hypothesis that replacing Ala19 in growth hormone-releasing factor (GRF) with more hydrophobic residues will increase growth hormone releasing activity, four GRF analogs were prepared and tested. The molecules were made by substituting Val, Ile, or Leu at position 19 of [Thr2,Ala15,Leu27]bGRF(1-29)NH2. The compounds were evaluated for growth hormone (GH) releasing activity in vitro (rat anterior pituitary cells) and in vivo (steers). Additionally, their half-life in vitro was determined in bovine plasma, and their secondary structure was examined by circular dichroism. In pituitary cells, peptides with substitutions at position 19 had the following potencies: Ala (native), 0.37; Val, 1.16; Ile, 0.37; Leu, 0.043. When assayed in steers as a single iv bolus, over a 2-h period, the compounds gave the following integrated GH response: Ala, 2.75; Val, 2.67; Ile, 2.57; Leu, 1.55. Only the Leu analog was statistically different from the other three (p = 0.05). In bovine plasma, the half-lives (hours) were as follows: Ala, 4.9; Val, 6.6; Ile, 12.3; Leu, 14.7. In phosphate buffer the compounds were calculated to have the following percent helical content: Ala, 26; Val, 21; Ile, 27; Leu, 32. For these analogs, helicity in aqueous buffer is inversely related to their in vitro activity. Using a linear multiple regression model, the plasma half-life of the analogs positively correlated (r2 = 0.999) with both the hydrophobicity of the residue at position 19 and the helicity of the analog. Although the Val analog had both increased inherent activity and increased plasma stability in vitro compared to the Ala analog, in this study we were unable to demonstrate an increase in activity in vivo. The in vivo GH releasing activity of the analogs was not simply related to a combination of their intrinsic GH releasing activity and their in vitro plasma half-life. This suggests that in vivo additional factors are moderating the expression of activity.

Degradation of growth hormone releasing factor analogs in neutral aqueous solution is related to deamidation of asparagine residues. Replacement of asparagine residues by serine stabilizes.

The incubation of a solution of the human growth hormone releasing factor analog, [Leu27] hGRF(1-32)NH2 at pH 7.4 and 37 degrees, resulted in extensive degradation of the sample. The major degradation products were identified as the peptides [beta-Asp8, Leu27] hGRF(1-32)NH2 and [alpha-Asp8, Leu27] hGRF(1-32)NH2, produced by deamidation of the Asn8 residue. When tested as growth hormone (GH) secretagogues in cultured bovine anterior pituitary cells, [beta-Asp8, Leu27] hGRF(1-32)NH2 was estimated to be 400-500 times less potent than the parent Asn8 peptide, while [alpha-Asp8, Leu27] hGRF(1-32)NH2 was calculated to be 25 times less potent than the parent Asn8 peptide. Three additional analogs of [Leu27] hGRF(1-32)NH2 containing either Ser or Asn at positions 8 and 28 were prepared and evaluated for their GH releasing activity and stability in aqueous phosphate buffer (pH 7.4, 37 degrees). Based on disappearance kinetics, [Leu27] hGRF(1-32)NH2 had a half-life of 202 h while the other analogs had the following half-lives: [Leu27, Asn28] hGRF(1-32)NH2 (150 h); [Ser8, Leu27, Asn28] hGRF(1-32)NH2 (746 h); and [Ser8, Leu27] hGRF(1-32)NH2 (1550 h). After 14 days, incubated samples of the Asn8 analogs lost GH releasing potency, while the Ser8 analogs retained full potency. The potential for loss of biological activity brought about by deamidation of other engineered peptides and proteins should be considered in their design.

Problems associated with use of the benzyloxymethyl protecting group for histidines. Formaldehyde adducts formed during cleavage by hydrogen fluoride.

The use of N alpha-tert.-butyloxycarbonyl-N pi-benzyloxymethylhistidine in peptide synthesis resulted in significant levels of several different side products attributable to the generation of formaldehyde during the hydrogen fluoride cleavage reaction. Methylated impurities in a decapeptide were isolated and identified. These methylated impurities were attributed to the use of the benzyloxymethyl protecting group for the histidines, since the impurities did not form when the dinitrophenyl protecting group was used. Also, peptides containing benzyloxymethyl-protected histidines in addition to N-terminal cysteines quantitatively yielded their respective N-terminal thiazolidine derivatives upon isolation from standard hydrogen fluoride cleavage mixtures. Thiazolidine ring formation was circumvented by including in the cleavage reaction a formaldehyde scavenger such as cysteine hydrochloride or resorcinol.