EVEROLIMUS – TEARING mTOR CASCADES

The mTOR pathway

One of the more central proteins in the many sequences of pathways that regulate growth patterns in cellular system is the “mammalian target for rapamycin”. Discovered by accident during the screening of extracts from plant and bacterial origin for biological activity this protein has proved to be highly complex in its activity. In an initial investigation it was demonstrated that this protein was essential for life itself, murine models mutated to silence this protein genetically did not survive the embryonic stage. [1;2] Structurally this protein resembles a lipid kinase but it also demonstrates significant functions similar to the protein kinase super families [3]. This protein contains more than one binding domain demonstrating kinase activity. These domains can bind with several possible ligands which trigger several different pathways. The mTOR protein can initiate scaffold building for DNA repair, to regulate other pathways in gene transcription and growth processes and regulate its own activity. mTOR signaling can be indirect as a results of a pathway trigger but also direct from extracellular signals. [4].

Everolimus: Some Properties and important usages

Everolimus was developed in response to the clinical evidence supplied by Rapamycin. With resistance to rapamycin becoming more evident and therapeutically limiting the dimethoxy cyclohexane side chain on the 28 atom cyclic structure that makes up Rapamycin was altered to a 2-((2-methoxycyclohexyl)oxy)ethanol. Like Rapamycin Everolimus was determined to be a potent immunosuppressant and it is used extensively to suppress transplant rejection. Everolimus is manufactured by a semisythesis procedure from Rapamycin [6]. Everolimus is being developed by Novartis; it is marketed under the trade names of Afinitor, Zortress or Certican, but also researched under the code OSI-RAD001 or SDZ-RAD. The Everolimus structure is based on a polyketide class of natural compounds with a molecular weight of 958.224. Everolimus solubility in buffer preparations is at a maximum of 10-20 µM but Everolimus is soluble in both DMSO and ethanol to a maximum 100 mg/ml concentration. Everolimus is specifically targeted against the mTOR1 protein but not the mTOR2 protein with the Everolimus IC50 for mTOR1 in the 1 -2 nM range. Everolimus stability is programmed for its lyophilized form which is stable at -20^oC or below and can stored under these conditions for a maximum of 2 years. In solution Everolimus can be kept at -20°C for a maximum of 3 months with minimal thawing. It is advised that aliquots of solution are stored for daily use and remnants discarded rather than refrozen. Researchers can buy Everolimus a 50 mg vial from a variety of Everolimus suppliers although Everolimus cost is reliant on the supplier of the product. Everolimus price can range from $179 up to $800 for the sterilized product.

Everolimus: Clinical status

Everolimus is 1^st generation derivative of rapamycin that has been demonstrated to be an inhibitor of  mTOR1 (FRAP1)[7] and also a substrate for CYP3A4[8]. With close similarity in structure to rapamycin Everolimus has been tested in a variety of different diseases but has proved successful as an immunosuppressor in Liver [9], Cardiac [10] and Kidney[11] transplant rejections in relation to graft vascular illness[12]. Everolimus clinical trials are extensive with various degrees of success and failure. In metastatic breast cancer [13-15] Everolimus is or has been trialed in combination with Erlotinib, Docetaxel[16], cisplatin and paclitaxel or Letrozole or Fulvestrant or Tratuzumab[17], initial phase 1 and II trials are showing promising results in the HER2 over expressing patient subgroup. In combination with Vatalanib, erlotinib[18] or Bevacizumab [18;19] and Panitumumab or Fluorouracil, or Capecitabine[20]or Leucovorin or cisplatin [21] or Panitumumab, and Oxaliplatin Everolimus has been investigated with regard to advanced solid tumors[22-24]( Everolimus, carcinoids). In metastatic melanoma, kidney cancer, colorectal cancer [19], Renal cell carcinoma [25;26], hepatocellular cancer [27] Everolimus is or has been investigated as a single agent or in combination with Bevacizumab[28], Sorafenib [29] or Lenalidomide. As a single agent or in combination with cisplatin or imatinib[30] Everolimus is or has been investigated in advanced gastric cancer (Everolimus gastric cancer) while in Kidney cancer Everolimus has been investigated as a single agent [31-36]. As a single agent or in combination with Gleevec and hydroxyurea[37] or in combination with Temozolamide[38] / Bevacizumab Everolimus has been investigated in gliosarcoma, Low grade Gliomas and glioblastoma multiforme. This is but a small sample of the investigation in which Everloimus has been trialed.

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