Aspartate carbamoyltransferase activity, drug concentrations, and pyrimidine nucleotides in tissue from patients treated with N-(phosphonacetyl)-L-aspartate

Novel Highlight in Malarial Drug Discovery: Aspartate Transcarbamoylase

Malaria remains one of the most prominent and dangerous tropical diseases. While artemisinin and analogs have been used as first-line drugs for the past decades, due to the high mutational rate and rapid adaptation to the environment of the parasite, it remains urgent to develop new antimalarials. The pyrimidine biosynthesis pathway plays an important role in cell growth and proliferation. Unlike human host cells, the malarial parasite lacks a functional pyrimidine salvage pathway, meaning that RNA and DNA synthesis is highly dependent on the de novo synthesis pathway. Thus, direct or indirect blockage of the pyrimidine biosynthesis pathway can be lethal to the parasite. Aspartate transcarbamoylase (ATCase), catalyzes the second step of the pyrimidine biosynthesis pathway, the condensation of L-aspartate and carbamoyl phosphate to form N-carbamoyl aspartate and inorganic phosphate, and has been demonstrated to be a promising target both for anti-malaria and anti-cancer drug development. This is highlighted by the discovery that at least one of the targets of Torin2 – a potent, yet unselective, antimalarial – is the activity of the parasite transcarbamoylase. Additionally, the recent discovery of an allosteric pocket of the human homology raises the intriguing possibility of species selective ATCase inhibitors. We recently exploited the available crystal structures of the malarial aspartate transcarbamoylase to perform a fragment-based screening to identify hits. In this review, we summarize studies on the structure of Plasmodium falciparum ATCase by focusing on an allosteric pocket that supports the catalytic mechanisms.

Phase II trial of N-(phosphonacetyl)-L-aspartate (PALA), 5-fluorouracil and recombinant interferon-alpha-2b in patients with advanced gastric carcinoma

The aspartate transcarbamoylase inhibitor, N-(phosphonacetyl)-L-aspartate (PALA), synergistically enhanced the cytotoxicity of a combination of 5-fluorouracil (5-FU) and interferon-alpha (IFN) against human colon cancer cell lines in vitro. To test the efficacy of this combination in the clinical setting, patients with locally advanced or advanced gastric carcinoma were treated with the combination of PALA, 5-FU and IFN (PFI). Patients were required to have biopsy-proven disease beyond the scope of surgical resection, measurable disease, no prior chemotherapy, adequate bone marrow, renal and hepatic function, to be fully ambulatory and to have given informed consent. Drug was administered as follows: PALA, 250 mg/m2, 15 min i.v. infusion, days 1, 15, 22, 29, and then weekly; 5-FU, 750 mg/m2 daily x 5 as a continuous i.v. infusion beginning day 2, then at 750 mg/m2 days 16, 23 and 30, then weekly; IFN, 9 MU subcutaneously three times per week beginning day 2. There were 22 patients enrolled. The major toxicities were fatigue and associated neurotoxicity, with acceptable gastrointestinal and haematological toxicities. There was one complete responder (5%) and 3 partial responders (14%); two of these responses were durable (> 3 years). Despite this modest clinical activity, other regimens for advanced gastric cancer such as FAMTX and ELF appear to have greater activity with comparable toxicity.

Modulation of metabolism and cytotoxicity of cytosine arabinoside with N-(phosphon)-acetyl-L-aspartate in human leukemic blast cells and cell lines

Cytosine arabinoside (Ara-C) activation to cytosine arabinoside triphosphate (Ara-CTP) and subsequent incorporation into DNA is regulated by the pyrimidine nucleotides UTP, CTP and dCTP. Inhibition of the de novo synthesis of these pyrimidine nucleotides by N-(phosphon)-acetyl-L-aspartate (PALA) may enhance the cytotoxicity of Ara-C. We therefore studied the effect of PALA on Ara-C cytotoxicity and on Ara-CTP accumulation and incorporation into DNA on cell lines and patient samples. Fifty micromolar PALA increased the growth inhibitory effect of Ara-C in U937 cells several fold both with pre- and coincubation. Ara-C cytotoxicity was not potentiated by PALA in Hl60 cells. However, coincubation with PALA did not enhance Ara-CTP accumulation both in HL60 and U937 cells, nor affect Ara-C incorporation into DNA. Ara-C cytotoxicity to leukemic blast cells from 11 untreated patients with different types of leukemia was only modulated to a small extent by high PALA concentrations in only two cases. Ara-CTP accumulation in leukemic blast cells varied from non-detectable levels to 200 pmol/10(6) cells. Fifty micromolar PALA enhanced the accumulation of Ara-CTP significantly in only one patient with no apparent effect on UTP and CTP levels. Raising PALA to 500 microM decreased UTP and CTP levels to 50% but had no effect on Ara-CTP levels. In conclusion, modulation by PALA of Ara-C cytotoxicity and metabolism is limited in leukemic cells, both in culture and from patients. This suggests the possibility for selective modulation of other agents by PALA on non-hematological cells.

Contribution of de-novo and salvage synthesis to the uracil nucleotide pool in mouse tissues and tumors in vivo

The relative contribution of de-novo and salvage synthesis to tissue pyrimidine nucleotide pools is an important parameter in the rational design of anti-pyrimidine therapies, but has not been measured in vivo. We have measured the contribution of de-novo synthesis to the total acid-soluble uracil nucleotide pool in mouse tissues by analysis of the incorporation of label after intra-peritoneal infusion of L-[15N]alanine. The contribution of salvage synthesis was measured by the incorporation of radiolabel after intravenous infusion of [14C]uridine. The results show that de-novo synthesis makes the larger contribution to the intestine uracil nucleotide pool, salvage synthesis makes the larger contribution to the kidney pool, and de-novo and salvage synthesis make roughly equal contributions to the liver pool. In tumors studied (L1210, P388, B16, Nettesheim), the contribution of de-novo synthesis was at least five times the contribution of salvage synthesis. The measurements were repeated 24 hours after a 400-mg/kg dose of N-phosphonacetyl-L-aspartic acid. De-novo synthesis was substantially inhibited in all tissues and tumors after this treatment, although significant residual activity was observed in the intestine and L1210 cells. Nettesheim carcinoma was the only tumor or tissue to show a significant increase in salvage synthesis after N-phosphonacetyl-L-aspartic acid treatment.

The role of low-dose PALA in biochemical modulation

Phosphonacetyl-L-aspartate (PALA) is a rationally-synthesized analog of the transition-state intermediate in the formation of carbamyl aspartate from carbamyl phosphate and aspartic acid by aspartate carbamyl transferase (ACTase). PALA is thus a potent inhibitor of the enzyme (Ki about 10(-8) M for ACTases of various origins), which in whole cells blocks the de novo synthesis of pyrimidines. In vivo, low doses of PALA inhibit whole body pyrimidine synthesis. While this action is cytotoxic in vitro, extensive human testing demonstrates that PALA alone is devoid of selective antitumor activity. Recent interest in the therapeutic action of PALA derives from the demonstration that its action potentiates the cytotoxicity of several cytotoxic drugs, notably 5-fluorouracil (5-FU). Results from clinical trials of PALA and 5-FU in combination in colorectal cancer suggest that biochemical modulation with regimens which follow the principles determined in preclinical studies may enhance the efficacy of current therapy.

Metabolism and action of amino acid analog anti-cancer agents

The preclinical pharmacology, antitumor activity and toxicity of seven of the more important amino acid analogs, with antineoplastic activity, is discussed in this review. Three of these compounds are antagonists of L-glutamine: acivicin, DON and azaserine; and two are analogs of L-aspartic acid: PALA and L-alanosine. All five of these antimetabolites interrupt cellular nucleotide synthesis and thereby halt the formation of DNA and/or RNA in the tumor cell. The remaining two compounds, buthionine sulfoximine and difluoromethylornithine, are inhibitors of glutathione and polyamine synthesis, respectively, with limited intrinsic antitumor activity; however, because of their powerful biochemical actions and their low systemic toxicities, they are being evaluated as chemotherapeutic adjuncts to or modulators of other more toxic antineoplastic agents.

Binding of radiolabeled N-(phosphonacetyl)-L-aspartate to aspartate transcarbamylase from Ehrlich ascites tumor cells

Binding of N-(phosphonacetyl)-[3H]L-aspartate (PALA) to aspartate transcarbamylase (ATCase, EC 2.1.3.1) in crude extracts from Ehrlich ascites tumor cells was examined. At pH 7.4, the dissociation constant was 1.39 +/- 0.22 nM; the maximal binding capacity indicated an average intracellular ATCase concentration of 0.13 microM. The presence of phosphate, MgCl2, or CaCl2 increased the apparent dissociation constant for [3H]PALA without altering the maximal binding capacity. Phosphate, a product of the ATCase reaction, probably acts as a competitor for the PALA binding site; Mg2+ and Ca2+ may inhibit [3H]PALA binding by forming a chelate which reduces the effective concentration of the free [3H]PALA. Carbamyl phosphate was a relatively weak inhibitor of [3H]PALA binding in N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) buffer alone. Addition of NaF, an inhibitor of nonspecific phosphatases, decreased the carbamyl phosphate "Ki" as an inhibitor of [3H]PALA binding to 7 microM, a value close to the Km. NaF appears to act as an inhibitor of carbamyl phosphatase activity present in the cell extract. A first-order dissociation rate constant of 0.050 +/- 0.004 min-1 (T1/2 = 14 min) was determined by following displacement of [3H]PALA with excess unlabeled PALA. The dissociation rate was strongly temperature dependent. A second-order rate constant of 3.6 X 10(7) liters mol-1 min-1 was calculated from this rate constant and the dissociation constant. Using these kinetic constants, a simple computer model predicted that PALA binding to ATCase is 95% complete within 14 min under the conditions of the assay; at intracellular ATCase concentrations, binding is slightly faster. These results are discussed in the context of both the kinetics of inhibition and the reversal of inhibition of pyrimidine synthesis within the intact cell.