Design and preparation of serine-threonine protein phosphatase inhibitors based upon the nodularin and microcystin toxin structures. Part 3

Nodularin and microcystins are complex natural cyclic isopeptidic hepatotoxins that serve as subnanomolar inhibitors of the eukaryotic serine-threonine protein phosphatases PP1 and PP2A, enzymes that are intimately involved in controlling cellular metabolism. Previously we described a solution-phase synthesis of stripped-down nodularin analogues; cyclo[-beta -Ala-(R)-Glu-alpha -OMe-gamma -Sar-(R)-Asp-alpha -OMe-beta-(S)-Phe-] 3 and cyclo[-(3R)-3-hydroxymethyl-beta -Ala-(R)-Glu-alpha -OMe-gamma -Sar-(R)-Asp-alpha -OMe-beta-(S)-Phe-] 5. The synthetic strategy was designed to allow post-macrocyclisation elaboration. Here we examine alternative methods for introducing diversity and achieving macrolactamisation and compare the relative efficiency of solution- vs. solid-phase peptide syntheses of the macrocycles. Syntheses and the biological activities of the macrocycles cyclo{-[(2R)-alpha -4-benzylpiperidinylamido-Asp]-beta-[(R)-Glu]-gamma -Sar-[(R)-Asp]-beta-(S)-Phe-} 29 and cyclo{-(2S)-Phe-[(2R)-alpha -4-benzylpiperidinylamido-Asp]-(R)-Glu-gamma-(S)-Pro-beta-(R)-Asp-} 65 are compared. Both compounds contain sufficient side-chain functionality to interact with a hydrophobic groove at the enzyme active site. The proline containing analogues 30, 31 (R-3 = CH3) where sarcosine is replaced in macrocycles 3 and 4, were also synthesised in order to correlate conformational properties with biological activity. In accord with predictions macrocycles 29 and 65 were found to be weak inhibitors of PP1 with IC50 2.9 and 2.7 mM respectively.