Farnesyl inhibition

Rheb Elevated in many tumor cells, Rheb may be the critical target by which farnesyl transferase inhibitors inhibit tumor growth... 

The protein Ras, an important intracellular signal transducer, is crucially involved in the development of tumor growth. The farnesylation of Ras, catalyzed by the enzyme Ras-farnesyl-transferase, is essential to its proper functioning in the normal and in the transformed state. Therefore, the inhibition of Ras lipidation has become a promising target for the development of new classes of anti-tumor agents. This review focuses on the different classes of Ras-farnesyl-transferase inhibitors and compares their biological properties and modes of action in vitro as well as in vivo. 

The finding that lipid-modification of Ras, in particular farnesylation, is crucial to its biological activity led to the idea that inhibiting the enzymes re-... 

The less polar methyl ester 2 as prodrug showed better results in vivo and inhibits both farnesylation of the Ras protein and growth of Ras-transformed cells, whilst proliferation of Raf- or Mos-transformed cells was not influenced. Growth of human pancreatic adenocarcinoma cells with mutated K-Ras, c-Myc and p53 genes was inhibited by application of 2. If the compound is administered over a period of 5 days to mice with implanted Ras-dependent tumors, tumor growth can be reduced by up to 66% compared to untreated mice, whereas application of the antitumor antibiotic doxorubicin only resulted in 33% reduction under the same conditions. It is particularly noteworthy that treatment with the /1-turn mimetic - in contrast to treatment with doxorubicin - was without any visible side effects, such as weight loss. 

However, 2 also affected the regulation of actin stress fiber formation [19]. Rho proteins are involved in the regulation of various cytoskeletal structures, and RhoB is believed to be one of the prime targets of FTase inhibitors. Rho B is apparently both geranylgeranylated and farnesylated [20, 21]. If cells were treated with 2, vesicular localization of Rho B was inhibited. Thus 2 may also inhibit the farnesylation of Rho B, thereby interfering with actin stress fiber formation [22]. 

Further modifications of the CAAX tetrapeptide structure led to inhibitor 3 which blocked H-Ras farnesylation with an IC50 of 11 nmol/1 [23]. Tumor cell lines expressing mutant H- and N-Ras were most sensitive against this compound that inhibits tumor growth of EJ-1 human bladder carcinomas by about... 

The central unit of these peptidomimetics imitates a /1-turn and brings the NH2-terminus of the cysteine analogue and the CO OH terminus of the methionine in spatial proximity these can then complex the Zn2+ ion which is essential for activity of the FTase [26]. The free acid 7 inhibits the enzyme with an IC50 value of 1 nmol/1, whilst in intact cells the methyl ester 8, despite its weaker in vitro activity, is significantly more potent because it can penetrate the plasma membrane better due to its lower polarity. This property can be used to convert the morphology of H-Ras-transformed cells back to the normal form and to inhibit growth of these cells, whereas the substance shows no effect on Src-transformed and untransformed rat fibroblasts. The inhibitor therefore acts selectively on transformed cells and does not influence growth of normal cells. This result is noteworthy because farnesylation of the wild type H-Ras protein... 

There are several possible explanations to account for this apparent lack of toxicity. Some geranylgeranylated Ras-related proteins might compensate for the loss of Ras function (see, e.g., [46]). Alternatively inhibition of farnesyl transferase may reduce Ras activity below the level required for transformation, yet allow sufficient Ras activity for maintaining normal cell viability [47]. Alternatively, a different signaling pathway may be activated when Ras is not anchored to the plasma membrane. 

Angibaud et al. carried out thorough studies on the farnesyl protein transferase inhibitory activity of substituted azoloquinolines <2003BML4365>. These authors found that some tetrazolo[l,5-a]quinolines 161 (Scheme 38) are promising agents for oral in vivo inhibition. 

To function, Ras must be attached to the plasma membrane. Translocation from the cytoplasm to membrane requires a series of posttranslational modifications that begin with farnesylation of the cysteine residue, the fourth amino acid residue from the C terminus of the protein, by famesyl protein transferase (FPTase) (64). Attachment of the hydrophobic 15-carbon lipid farnesyl group allows Ras molecule insertion into the plasma membrane and is crucial for Ras signaling activity and transformation properties. As farnesylation is required for oncogenic Ras function, FPTase inhibitors (FTIs) are obvious candidate antineoplastic agents. Several drugs that inhibit Ras farnesylation are at various stages of clinical development (65). 

A.2.2 Destabilization of Cationic Intermediates The electronegative character of a fluorinated substituent can be used to inhibit the development of the positive charge in a biological process when this latter involves a positively charged transition state. This approach has been used to perform mechanistic studies on the enzymatic farnesyl transfer involved in the synthesis of isoprenoids. The presence of fluorine atoms significantly decreases the transfer rates of the isopentenyl pyrophosphate catalyzed by the diphosphate famesyltransferase (FPPase) or by the farnesyl-transferase protein (FTPase). ... 

Alendronate Suppress the activity of osteoclasts in part via inhibition of farnesyl pyrophosphate synthesis Inhibit bone resorption and secondarily bone formation Osteoporosis, bone metastases, hypercalcemia Adynamic bone, possible renal failure, rare osteonecrosis of the jaw... 

Chen X, Hasuma T, Yano Y, Yoshimata T, Morishima Y, Wang Y, Otani S. 1997. Inhibition of farnesyl protein transferase by monoterpene, curcumin derivatives and gallotannin. Anticancer Res 17 2555-2564. 

In addition to Ras, a number of other proteins are known to be substrates for FTase (many of unknown identity or function, determined by 2-D gel shift experiments in the presence and absence of FTI) these may also play a part in determining relative susceptibility to FTase inhibition.46 In particular, RhoB, a farnesylated protein involved with the formation of actin stress fibers and cytoskeletal organization, has been implicated in the growth-inhibitory consequences of FTase inhibition.47,48 Another study using the FTI 19 found that growth inhibition in soft agar of a panel of human tumor cell lines correlated with the mutational status of the H-and N-Ras proteins, but not with K-Ras mutations.49 In this study also, cells with genetic defects in addition to ras were sensitive to FTase inhibition. 

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