This is a mini paper-review of sorts. A few posts ago I described work by Robert Gatenby’s work showing that using lower doses of chemotherapy could potentially lead to more durable antitumour responses. Today I will review some rather intriguing findings using a drug called decitabine, again starting off with the premise that lower doses might be better than higher doses.
Drugs like Azacitidine and Decitabine are passive hypomethylating agents – they are analogs of cytosine – one of the four bases that make up DNA, and react with DNA methyltransferase 1, which carries out DNA methylation and traps it. With no free enzyme to copy methylation states to daughter cells, methylation is progressively lost, with the hope being that genes that have to be silenced by methylation for cancer cells to survive/stay undifferentiated are reactivated, resulting in the castration of a cell’s malignant potential.
These drugs have found extremely limited use due to very high toxicity – being limited to haematological conditions like Myelodysplastic syndrome, but people noticed that there were delayed effects that appeared to be independent of the cytotoxicity of high doses. This, combined with a lung cancer trial where low doses in combination with an HDAC inhibitor showed some good results led researchers to investigate what treating cells with extremely low, normally subtoxic doses of these agents could do.
They found that the agents at those doses could blunt the ability of treated leukamia cell lines to form colonies in glass dishes (that’s a standard assay we use to test how effective an anticancer drug might be) and to induce leukaemia when injected into NOD/SCID mice (which lack an immune system and will therefore accept grafts of human cancers). Importantly, these doses did not hit the colony forming ability of normal blood-forming cells.
They did similar experiments with 5-Aza and Decitabine using a breast cancer cell line (MCF-7) and primary breast cancer tissue xenografts. Whatever was true for leukaemia cell lines was also true for these samples. They did a range of primary and secondary xenografts, wherein cells were treated with the drug for 72 hours at low doses (replaced daily) and left for 7-14 days before being put into mice, and secondary transplantations were made from these. There were potent long term effects brought about by drug treatment compared to untreated cells.
In primary breast cancer samples, they found that colony formation was impaired and treatment with Azacitidine depleted cells in the stem-like cell compartment (CD44+, CD24-). They profiled methylation and expression changes using arrays in treated cell lines and found a host of changes associated with increased expression of genes that suppress the cell cycle proper and entry into it.
I’ve tried using the drug on some of the cell lines I’ve worked on in the past and saw similar results in how treatment with a very low dose of decitabine could, upon transient exposure, result in cells that show divergent growth properties. Clearly, decitabine might be a promising addition to therapy in a broad spectrum of tumour types, especially if there are driver methylation events that drive cell survival. The existence of such methylation states will be the subject of another blog post soon in the future.
Paper Reference –
Tsai HC, Li H, Van Neste L, Cai Y, Robert C, Rassool FV, Shin JJ, Harbom KM,
Beaty R, Pappou E, Harris J, Yen RW, Ahuja N, Brock MV, Stearns V, Feller-Kopman D, Yarmus LB, Lin YC, Welm AL, Issa JP, Minn I, Matsui W, Jang YY, Sharkis SJ,Baylin SB, Zahnow CA. Transient low doses of DNA-demethylating agents exert durable antitumor effects on hematological and epithelial tumor cells. Cancer Cell. 2012 Mar 20;21(3):430-46. doi: 10.1016/j.ccr.2011.12.029. PubMed PMID:22439938; PubMed Central PMCID: PMC3312044.
Ankur ‘Exploreable’ Chakravarthy.