Inhibitors of FTase, GGTase

Definition
Farnesyltransferase inhibitors (FTIs) are a group of drugs that selectively inhibit the enzyme farnesyltransferase (FTase) that is responsible for the transfer of a farnesyl group to Ras and other proteins involved in signaling concerning cell transformation and survival.

Geranylgeranyl transferase type 1 inhibitor can affect critical cellular processes such as inhibition of platelet-derived growth factor receptor tyrosine kinase phosphorylation and growth arrest of human neoplastic cells in G1, presumably through inhibition of RhoA geranylgeranylation.

Methyltransferase inhibitors have ability to inhibit hypermethylation, restore suppressor gene expression and exert antitumor effects.

Structural Characteristics
Farnesyl protein transferase inhibitors - Initial approaches to FT inhibition involved the use of general inhibitors of isoprenylation. Synthesis of farnesyl groups can be blocked by the HMG CoA (i.e., 3-hydroxy-3-methylglutaryl-coenzyme A) reductase inhibitors, such as lovastatin, and the mevalonate pyrophosphate decarboxylase inhibitor phenylacetate1.

The FTIs fall into three main classes: 1) the CAAX competitive inhibitors, 2) the FPP competitive inhibitors and 3) the bisubstrate inhibitors.

Peptidomimetic GGTIs have been described. These include aminobenzoic acid derivatives such as GGTI-298 and GGTI-2154 and benzoyleneurea-based compounds. Studies with these compounds have revealed a number of consequences of cellular exposure to a GGTI. Administration of GGTI to cells can cause cell cycle arrest at G0/G1, and this effect appears to be mediated by inactivation of CDK2/4 through the p21/p15 kinase inhibitors downstream of Rho. GGTIs are also potent stimulators of apoptosis in both normal and transformed cell lines2, 3.

DNA methyltransferase inhibitors - DNA methylation inhibitors have demonstrated the ability to inhibit hypermethylation, restore suppressor gene expression and exert antitumor effects. Four inhibitors, which are analogs of the nucleoside deoxycitidine, have been clinically tested: 5-azacytidine, 5-aza-2'-deoxycytidine, 1-ß-D-arabinofuranosyl-5-azacytosine and dihydro-5-azacytidine. The first two have demonstrated encouraging antileukemic activity but little activity in solid tumors, while the latter two are no longer under study due to lack of efficacy4.

Mode of Action
Farnesyl and Geranylgeranyl Transferase Inhibitors Induce G1 Arrest by Targeting the Proteasome - In a study, using breast tumor models, it has been shown those agents possessing a lactone moiety, including statins (such as lovastatin) and the isoprenoid inhibitors (such as FTI-277 and GGTI-298), mediate their cell cycle inhibitory activities by blocking the chymotrypsin activity of the proteasome. This results in the accumulation of cyclin-dependent kinase inhibitors p21 and p27 with subsequent G1 arrest. Cells devoid of p21 were refractory to the growth-inhibitory activity of lovastatin, FTI-277, and GGTI-298. However, in p21 null cells, isoprenylation of key substrates of farnesyl transferase (such as Ras) and of geranylgeranyl transferase (such as RAP-1) were inhibited by FTI-277 and GGTI-298, respectively, suggesting that although both these isoprenoid inhibitors reached and inhibited their intended targets, inhibition of the isoprenylation of Ras and RAP-1A are not sufficient to mediate G1 arrest5.

DNA methyltransferase inhibitors - These drugs act by preventing methylation. Decitabine, for example, when incorporated into DNA, covalently link with DNMT which may induce cell death by obstructing DNA synthesis. They may also induce DNA damage through structural instability at the site of incorporation6.

Functions
Farnesyltransferase and geranylgeranyltransferase I inhibitors upregulate RhoB expression – it has been demonstrate that the novel antitumor agents farnesyltransferase inhibitors (FTIs) and geranylgeranyltransferase I inhibitors (GGTIs) upregulate RhoB expression in a wide spectrum of human cancer cells including those from pancreatic, breast, lung, colon, bladder and brain cancers. RhoB induction by FTI-277 and GGTI-298 occurs at the transcriptional level and is blocked by actinomycin D. Reverse transcription-PCR experiments documented that the increase in RhoB protein levels is due to an increase in RhoB transcription. Furthermore, treatment with FTIs and GGTIs of cancer cells results in HDAC1 dissociation, HAT association and histone acetylation of the RhoB promoter. Thus, promoter acetylation is a novel mechanism by which RhoB expression levels are regulated following treatment with the anticancer agents FTIs and GGTIs7.

DNA methyltransferase inhibitor, induces ATR-mediated DNA double-strand break responses, apoptosis, and synergistic cytotoxicity with doxorubicin and bortezomib against multiple myeloma cells - In a study, 5-azacytidine, a DNA methyltransferase inhibitor showed significant cytotoxicity against both conventional therapy-sensitive and therapy-resistant MM cell lines, as well as multidrug-resistant patient-derived MM cells, with IC50 of 0.8–3 µmol/L. Conversely, 5-azacytidine was not cytotoxic to peripheral blood mononuclear cells or patient-derived bone marrow stromal cells (BMSC) at these doses. 5-Azacytidine treatment induced DNA double-strand break (DSB) responses, as evidenced by H2AX, Chk2, and p53 phosphorylations, and apoptosis of MM cells. Further it has been shown that 5-azacytidine–induced DNA DSB responses were mediated predominantly by ATR, and that doxorubicin, as well as bortezomib, synergistically enhanced 5-azacytidine–induced MM cell death8.

References:
1.End DW (1999). Farnesyl protein transferase inhibitors and other therapies targeting the Ras signal transduction pathway. Invest. New. Drugs, 17:241–258.
2.Lerner EC, Qian Y, Hamilton AD, Sebti SM (1995). Disruption of oncogenic K-Ras4B processing and signaling by a potent geranylgeranyltranferase I inhibitor. J. Biol. Chem, 270:26770–26773.
3.Vasudevan A, Qian Y, Vogt A, Blaskovich MA, Ohkanda J, Sebti SM, Hamilton AD (1995). Potent, highly selective, and non-thiol inhibitors of protein geranylgeranyltransferase-I. J Med Chem, 42:1333–1340.
4.Goffin J, Eisenhauer E (2002). DNA methyltransferase inhibitors—state of the art. Annals of Oncology, 13:1699-1716.
5.Efuet ET, Keyomarsi K (2006). Farnesyl and Geranylgeranyl Transferase Inhibitors Induce G1 Arrest by Targeting the Proteasome. Cancer Research, 66(2): 1040-1051.
6.Juttermann R, Li E, Jaenisch R (1994). Toxicity of 5-aza-2'-deoxycytidine to mammalian cells is mediated primarily by covalent trapping of DNA methyltransferase rather than DNA demethylation. Proc Natl Acad Sci, 91(25): 11797–11801.
7.Delarue FL, Adnane J, Joshi B, Blaskovich MA, Wang DA, Hawker J, Bizouarn F, Ohkanda J, Zhu K, Hamilton AD, Chellappan S, Sebti SM (2006). Farnesyltransferase and geranylgeranyltransferase I inhibitors upregulate RhoB expression by HDAC1 dissociation, HAT association and histone acetylation of the RhoB promoter. Oncogene, 1;26(5):633-40.
8.Kiziltepe T, Hideshima T, Catley L, Raje N, Yasui H, Shiraishi N, Okawa Y, Ikeda H, Vallet S, Pozzi S, Ishitsuka K, Ocio EM, Chauhan D, Anderson KC (2007). 5-Azacytidine, a DNA methyltransferase inhibitor, induces ATR-mediated DNA double-strand break responses, apoptosis, and synergistic cytotoxicity with doxorubicin and bortezomib against multiple myeloma cells. Mol. Cancer. Ther. 6(6):1718–1727.