5'-Deoxy-5'-methylthioadenosine (MTA) is produced stoichiometrically during polyamine synthesis in mammalian cells, and is rapidly cleaved to adenine and methylthioribose-1-P by a ubiquitous MTA phosphorylase (MTAP). MTAP activity is the only source of endogenous adenine in cells, which is salvaged back to AMP to ATP pools. Discovery of MTAP deficiency in certain neoplasms led to the identification of the tumor suppressor gene CDKN2/pl6 linked to MTAP on chromosome 9p2l. Homozygous deletions of 9p occur in lymphoblastic leukemia, glioma, and non-small cell lung cancer (NSCLC). Due to the proximity of MTAP to CDKN2/pl6, MTAP deficiency occurs in these cancers harboring 9p deletions: 30% of primary NSCLC samples were found to be MTAP-deficient. MTAP-containing cells proliferate despite drugs that inhibit ATP synthesis, if MTA is available as a purine source. In contrast, MTAP-deficient cells cannot produce adenine for salvage, and are selectively killed by inhibitors of de novo ATP synthesis despite MTA. L-alanosine is an aspartate analogue that inhibits adenylosuccinate synthetase (ASS), the branch point enzyme in de novo AMP synthesis. The condensation product of L-alanosine and aminoimidazolecarboxylate ribonucleotide potently inhibits ASS at clinically tolerated dosages. However, past clinical trials failed to show significant activity of L-alanosine bolus therapy. In retrospect, none of the malignancies in which L-alanosine was tested exhibit homozygous CDKN2 deletions except rarely, and no selective effect on MTAP-deficient tumors could have been expected. Thus, L-alanosine has never been tested in MTAP-deficient cancer. Preliminary data show that MTAP-deficient malignant cell lines are intrinsically more sensitive to L-alanosine than histologically matched MTAP-positive lines. Supplying MTA selectively restores proliferation only in alanosine-treated cells that contain MTAP. These data support the novel hypothesis that L- alanosine may be selectively toxic to MTAP-deficient tumors, at doses not toxic to normal tissues able to salvage MTA-derived adenine. This hypothesis is successfully confirmed in a nude mouse NSCLC xenograft model, in which daily or continuous dosing of L-alanosine caused regression only of MTAP-deficient tumors. The goal of the proposed research is to test this hypothesis by Phase II clinical trial of L- alanosine in NSCLC patients with measurable disease, whose tumors are documented MTAP-deficient by genetic or immunochemical assays (Aim 1). 31P magnetic resonance spectroscopy to document tumor ATP depletion during therapy is also planned. Further confirmation of L-alanosine selectivity for MTAP-deficient tumors will be performed using nude mouse xenograft models (Aim 2).
| Schmid, M; Malicki, D; Nobori, T et al. (1998) Homozygous deletions of methylthioadenosine phosphorylase (MTAP) are more frequent than p16INK4A (CDKN2) homozygous deletions in primary non-small cell lung cancers (NSCLC). Oncogene 17:2669-75 |
