Enzymatic Ftase Inhibition

5.1. Enzymatic Structure Activity Relationships the BPHI series and is likely to be related to their FPP-competitive mode of inhibition (see Section 5.3.). Moreoverthese equipotent enzymatic inhibitions ofHa-Ras orKi-Ras farnesylation allows us to analyze enzymatic SAR data independently of the precise assay or substrate conditions.

5.1.1. Stereochemical Requirements

Chiral resolution of (±) II/1 emphasized the dextrogyre enantiomer (+) II/1 as being at least 100-fold more potent than the levogyre enantiomer (—) II/1, with IC50 values of 0.14 and 45 pM, respectively. At least for 3a-carboxylic acids, this dextrogyre-specific activity seems to be a general feature within the series, as exemplified by compounds (+) II/2, (+) II/25, and (+) II/53, which are 200-5000 times more potent than the corresponding levogyre enantiomers.

X-ray analysis of crystals from 4,9-ethano-2-[2-(2-methoxyphenyl)propen-2-oyl]-9-phenyl-2,3,3a,4,9,9a-hexahydro-1H-benzo[f]isoindol-3a-N-[1-(R)-phenylethyl]car-boxamide RPR 117101 (+) II/22, resulting from the condensation of RPR 115135 (+) II/2 with 1-(R)-phenylethylamine, demonstrated that the bioactive dextrogyre series of BPHI corresponds to the 3aS,4R,9R,9aS absolute configuration, with the 4,9-ethano bridge back andthe 3a-carboxylic acid in front (49). Moreover, in these crystals, the phenyl ring at position 9 and the phenyl ring ofthe side chain at position 2 are stacking together. Such a n-stacking conformation is also seen as the preferred one according to molecular modeling and energy minimization studies. This bioactive conformation will be assessed further by ongoing analysis of co-crystals obtained with RPR 130401 (+) II/25 and human FTase.

With (3-pyridyl)methylamides, the enzymatic stereoselection is much less crucial, because dextrogyre enantiomers (+) II/47 or II/48 are only 4-8 times more potent than their levogyre enantiomers. This discrepancy in the stereochemical requirements may reflect differences in the binding of carboxylic acids and (3-pyridyl)methylamides at position 3a.

5.1.2. S.A.R. on the Side-Chain at Position 2 (Table 2A)

The nature and the length of the side chain at position 2 also play a key role on the inhibitory potency. Either removal of the side chain at position 2 or its replacement by a 4-methoxy-butanoyl moiety leads to inactive compounds (data not shown), thus demonstrating the need for an aromatic ring on this side chain. The nature ofthe linking group is also crucial. Replacement of the arylacetyl moiety by an arylethyl group II/3, an arylcarbonylamino II/4 or an arylcarbonyloxy group (data not shown), or its shortening to a benzoyl group II/5, or its lengthening to an arylpropanoyl group (II/6) can significantly decrease FTase inhibition.

The position ofthe substituent on the phenyl ring ofthe side chain is also crucial. Simply shifting the methoxy group from the ortho to the meta II/7 or the para position (data not shown) also reduces activity. The precise role ofthe ortho methoxy group is not clear, as demonstrated by the drop in activity observed when it is removed II/8 or replaced by electron-withdrawing groups (data not shown), while some electron-donating groups usually lead to two- to fivefold less active analogues II/9-10. The introduction of a methylene group II/2, at the benzylic position, is the most favorable modification, thus leading to 2- to 3-fold increase in inhibition. More generally substitutions by alkyl or

Table 2A

Structure of Enzymatic and Cellular FTase Inhibitions of BPHI Modified At Position 2 II/1-11

co2h

Compound

Chemical structure R2

Enzymatic FTase inhibition Ha-Ras/ TCA (1) Ki-Ras/SPA (1)

Ras process, inhibition (2)

<±)II/1

C(0)-CH2-(2-Me0-Ph)

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