Potent long-acting alkylated analogs of growth hormone-releasing factor

Analogs of growth hormone-releasing factor-(1-29)-NH2 (GRF-29) involving derivatization with alkyl substituents at the N-terminus, with or without concomitant alkylation of basic amino acids within the peptide chain (lysine in positions 12 and 21), were synthesized using solid-phase methodology and tested for their ability to stimulate growth hormone release in pentobarbital and urethane-anesthetized rats and in 4-day primary cultures of rat anterior pituitary cells. N alpha-MeTyr1-GRF-29 was 40% as potent as GRF-29 15 min after injection of peptide into pentobarbital-anesthetized rats; N alpha-EtTyr1, Nle27-GRF-29 was 5 times more potent and N alpha-iPrTyr1-GRF-29 was 35 times more potent. Combining this latter modification with our previously reported superactive analog modification, D-Ala2, did not appreciably change potency. Analogs with larger alkyl groups, N alpha-cyclohexyl-Tyr1-GRF-29 were not superactive. However, N alpha-benzyl-Tyr1-GRF-29 was 73 times more potent than GRF-29. Most strikingly, the triisopropylated analog N alpha-iPrTyr1, N epsilon-iPr-Lys12,21-GRF-29 was 106 times more active. Several analogs were tested in time course assays. All were long-acting as compared to GRF-29, with the triisopropylated analog having the longest duration of activity. In vitro potencies among the analogs with the higher in vivo activities were only slightly increased (2-4 X) with respect to GRF-29. The observed increases in in vivo potencies presumably occur due to enhanced resistance to enzymatic degradation following alkylation or through some other pharmacokinetic effect due to the increased lipophilicity of these analogs.(ABSTRACT TRUNCATED AT 250 WORDS)