In the laboratory, this combination indeed showed increased temozolomide activity in tumor-cell culture in vitro and in animal models in vivo. However, a recently completed phase-II clinical trial with brain-tumor patients yielded mixed outcomes; while there was some improved therapeutic activity when O6-BG and temozolomide were given to patients with temozolomide-resistant anaplastic glioma, there seemed to be no significant restoration of temozolomide sensitivity in patients with temozolomide-resistant glioblastoma multiforme.Some efforts focus on engineering hematopoietic stem cells expressing the MGMT gene prior to transplanting them into brain-tumor patients. This would allow for the patients to receive stronger doses of temozolomide, since the patients hematopoietic cells would be resistant to the drug.High doses of temozolomide in high-grade gliomas have low toxicity, but the results are comparable to the standard doses.Two mechanisms of resistance to temozolomide effects have now been described: 1) intrinsic resistance conferred by MGMT deficiency (MGMTd) and 2) intrinsic or acquired resistance through MMR deficiency (MMRd). The MGMT enzyme is the first line of repair of mismatched bases created by TMZ. Cells are normally MGMT proficient (MGMTp) as they have an unmethylated MGMT promoter allowing the gene to be expressed normally. In this state, TMZ induced DNA damage is able to be efficiently repaired in tumor cells (and normal cells) by the active MGMT enzyme. Cells may grow and pass through the cell cycle normally without arrest or death. However, some tumors cells are MGMT deficient (MGMTd). This is most commonly due to abnormal methylation of the MGMT gene promoter and suppression of gene expression.
MGMTd has also been described to occur by promoter rearrangement. In cells with MGMTd, DNA damage by TMZ activates the next stage of repair in cells with a proficient Mismatch Repair enzyme complex (MMRp). In MMRp the MMR protein complex identifies the damage and causes cells to arrest and undergo death which inhibits tumor growth. However, if cells have combined MGMTd and MMR deficiency (MGMTd + MMRd) then cells retain the induced mutations and continue to cycle and are resistant to effects of TMZ. In gliomas and other cancers MMRd has now been reported to occur as primary MMRd (intrinsic or germline Lynch bMMRd) or as secondary MMRd (acquired - not present in the original untreated tumor). The latter occurs after effective treatment and cytoreduction of tumors with TMZ and then selection or induction of mutant MSH6, MSH2, MLH1, or PMS2 proteins and cells which are MMRd and TMZ resistant. The latter is described as an acquired resistance pathway with hotspot mutations in glioma patients (MSH6 p.T1219I). Other resistance pathways are also likely to exist. References
Further reading
External links
"Temozolomide". Drug Information Portal. U.S. National Library of Medicine. "Temozolomide (Temodal)". Cancer Research UK.