Increasing the therapeutic index of 5-fluorouracil and 6-thioguanine by targeting loss of MTAP in tumor cells

Our plain-English summary of Increasing the therapeutic index of 5-fluorouracil and 6-thioguanine by targeting loss of MTAP in tumor cells, by B. Tang, J.R. Testa, W.D. Kruger (Cancer Biology & Therapy, 2012).

The study

• Authors: B. Tang, J.R. Testa, W.D. Kruger
• Journal: Cancer Biology & Therapy
• Published: 2012;13(11):1082-90
• Cited by: 26 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Methylthioadenosine phosphorylase (MTAP), a key enzyme in the catabolism of 5'-deoxy-5'-methylthioadenosine (MTA), catalyzes the formation of adenine and 5-methylthioribose-1-phosphate. MTAP is expressed in all cells throughout the body, but a significant percentage of human tumors have lost MTAP expression, thereby making MTAP-loss a potential therapeutic target. Here, we have tested an MTAP-targeting strategy based on the idea that MTAP-expressing cells can be protected from toxic purine and uracil analogs by addition of MTA, but MTAP-deleted tumor cells cannot. Addition of as little as 10 μM MTA could entirely protect isogenic MTAP (+) , but not MTAP (-) , HT1080 cells from toxicity caused by the chemotherapy agents 6-thioguanine (6TG) or 5-fluorouracil (5FU). Inhibitor studies showed that MTA protection requires functional MTAP activity. Addition of adenine protected both MTAP (+) and MTAP (-) cells from 6TG and 5FU, consistent with the idea that adenine produced from the MTAP reaction competes with 6TG and 5FU for a rate limiting pool of phosphoribosyl-1-pyrophosphate (PRPP), which is required for the conversion of purine and uracil bases into nucleotides. Extracellular MTA can also protect mouse mesothelioma cells from killing by 6-TG or the drug L-alanosine in an MTAP-dependent manner. In addition, MTA can protect non-transformed MTAP (+) mouse embryo fibroblasts from 6TG toxicity. Taken together, our data suggest that the addition of MTA to anti-purine-based chemotherapy may greatly increase the therapeutic index of this class of drugs if used specifically to treat MTAP (-) tumors.

In plain English

This laboratory cell-culture study tested whether MTA (5'-deoxy-5'-methylthioadenosine) protects cells that still have the enzyme MTAP from the chemotherapy drugs 6-thioguanine (6TG) and 5-fluorouracil (5FU), while leaving MTAP-deleted tumour cells unprotected. In matched MTAP+ and MTAP- human HT1080 fibrosarcoma cells, 5 μM 6TG caused more than 85% growth inhibition in both, but adding 10 μM MTA nearly completely rescued the MTAP+ cells and gave no rescue of MTAP- cells, even at 6TG doses as high as 40 μM; 5FU showed the same behaviour. An MTAP inhibitor removed the protection, adenine protected both cell types, and thymidine protected neither. Among mouse mesothelioma lines from asbestos-treated Nf2+/- mice, 4 of 9 had lost MTAP expression; of 7 lines tested, MTA protected all five MTAP-positive lines and neither MTAP-null line, raising the 6TG IC50 34- to 1400-fold and the L-alanosine IC50 3.2- to 76.3-fold in MTAP+ cells. MTA also entirely reversed 6TG toxicity in non-transformed MTAP+ mouse embryo fibroblasts. The authors concluded that combining purine analogs with MTA might be useful in treating MTAP-deficient tumours.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: In vitro laboratory study using human and mouse cell lines (cell viability assays, western blots, enzyme activity measurement, pharmacologic inhibition)
• Who was studied: Cell lines: isogenic MTAP+ and MTAP- HT1080 human fibrosarcoma cells, 9 congenic mouse mesothelioma cell lines from asbestos-treated Nf2+/- mice (7 tested with drugs), and two primary mouse embryonic fibroblast lines
• Exposure: Drug treatments in culture: 6-thioguanine (0-40 μM), 5-fluorouracil (0-4 μM) and L-alanosine, with or without 10 μM MTA; also adenine, thymidine, and the MTAP inhibitor MT-DADMe-ImmA
• Main results:
  • In HT1080 cells without MTA, 5 μM 6TG caused more than 85% growth inhibition in both MTAP+ and MTAP- cells
  • Adding 10 μM MTA nearly completely rescued MTAP+ HT1080 cells but gave no rescue of MTAP- cells, with complete rescue even at 6TG doses up to 40 μM
  • With 5FU, the same pattern was seen: MTA protected MTAP+ but not MTAP- HT1080 cells
  • The MTAP inhibitor MT-DADMe-ImmA abolished MTA protection in MTAP+ cells, indicating MTAP enzyme activity is required
  • Adenine protected both MTAP+ and MTAP- HT1080 cells from 6TG and 5FU
  • Thymidine, used as a control, protected neither MTAP+ nor MTAP- cells
  • Of the 9 mouse mesothelioma lines from Nf2+/- mice, 6 showed loss of Cdkn2a/Arf expression and 4 of the 9 also showed loss of MTAP expression
  • In the 7 mesothelioma lines treated with 6TG, MTA protected all five MTAP-positive lines and neither of the two MTAP-null lines
  • In MTAP+ mesothelioma cells, MTA raised the 6TG IC50 by 34- to 1400-fold
  • All mesothelioma lines were sensitive to L-alanosine, but only the MTAP+ lines had this growth inhibition reversed by MTA
  • In MTAP+ mesothelioma cells, MTA raised the L-alanosine IC50 by 3.2- to 76.3-fold
  • Combining 6TG with L-alanosine in two MTAP+ lines gave no added inhibition and in fact less overall inhibition, which the authors read as antagonism
  • In non-transformed MTAP+ mouse embryo fibroblasts, 6TG inhibition at 20 μM or above was entirely reversed by 10 μM MTA
  • The same mouse embryo fibroblasts were not very sensitive to 5FU (40% decrease in cell viability at 20 μM) and this modest inhibition was not rescued by MTA
• The authors' conclusion: The authors concluded that their results support combining purine analogs with MTA to treat MTAP-deficient tumours, suggesting it may be possible to selectively kill MTAP-lacking tumour cells with high concentrations of 6TG or 5FU plus MTA
• Limitations the authors state:
  • The authors noted there is no guarantee that even high-dose 5FU or 6TG would be effective against a particular tumour type
  • The authors stated that further animal studies are needed to define how far the therapeutic window could be widened

Topics

Medical terms in this study, explained in our Lexicon: Fluorouracil (5-FU), MTAP, Chemotherapy, Mesothelioma, Fibroblast.

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