Characterisation of the Carboxypeptidase G2 Catalytic Site and Design of New Inhibitors for Cancer Therapy.

The enzyme carboxypeptidase G2 (CPG2) is used in antibody-directed enzyme prodrug therapy (ADEPT) to catalyse the formation of an active drug from an inert prodrug. Free CPG2 in the bloodstream must be inhibited before administration of the prodrug in order to avoid a systemic reaction in the patient. Although a few small-molecule CPG2 inhibitors have been reported, none has been taken forward thus far. This lack of progress is due in part to a lack of structural understanding of the CPG2 active site as well as the absence of small molecules that can block the active site whilst targeting the complex for clearance. The work described here aimed to address both areas. We report the structural/functional impact of extensive point mutation across the putative CPG2 catalytic site and adjacent regions for the first time, revealing that residues outside the catalytic region (K208A, S210A and T357A) are crucial to enzyme activity. We also describe novel molecules that inhibit CPG2 whilst maintaining the accessibility of galactosylated moieties aimed at targeting the enzyme for clearance. This work acts as a platform for the future development of high-affinity CPG2 inhibitors that occupy new chemical space and will advance the safe application of ADEPT in cancer treatment.

γ-Glutamyl hydrolase modulation and folate influence chemosensitivity of cancer cells to 5-fluorouracil and methotrexate.

BACKGROUND: γ-Glutamyl hydrolase (GGH) regulates intracellular folate and antifolates for optimal nucleotide biosynthesis and antifolate-induced cytotoxicity, respectively. The modulation of GGH may therefore affect chemosensitivity of cancer cells, and exogenous folate levels may further modify this effect. METHODS: We generated a novel model of GGH modulation in human HCT116 and MDA-MB-435 cancer cells and investigated the effect of GGH modulation on chemosensitivity to 5-fluorouracil (5FU) and methotrexate (MTX) at different folate concentrations in vitro and in vivo. RESULTS: Overexpression of GGH significantly decreased chemosensitivity of MDA-MB-435 cells to 5FU and MTX at all folate concentrations as expected. In contrast, in HCT116 cells this predicted effect was observed only at very high folate concentration, and as the folate concentration decreased this effect became null or paradoxically increased. This in vitro observation was confirmed in vivo. Inhibition of GGH significantly increased chemosensitivity of both cancer cells to 5FU at all folate concentrations. Unexpectedly, GGH inhibition significantly decreased chemosensitivity of both cancer cells to MTX at all folate concentrations. In both GGH modulation systems and cell lines, the magnitude of chemosensitivity effect incrementally increased as folate concentration increased. CONCLUSION: Modulation of GGH affects chemosensitivity of cancer cells to 5FU and MTX, and exogenous folate levels can further modify the effects.

Folate conjugase: two separate activities in human jejunum.

Previous perfusion studies of the human jejunum suggested that conjugated folate is hydrolyzed on the mucosal surface. The techniques of cell fractionation and DEAE and gel chromatography led to the identification of two separate folate conjugase activities in human jejunal mucosa: one membrane-bound and concentrated in the brush border, the other soluble and intracellular. These enzyme activities exhibit different pH optima, molecular weights, and inhibition characteristics. Folate conjugase in the brush border may accomplish the initial digestion of dietary pteroylpolyglutamates.

Biochemical and growth inhibition studies of methotrexate and aminopterin analogues containing a tetrazole ring in place of the gamma-carboxyl group.

The biological activities of novel analogues of methotrexate (MTX) and aminopterin (AMT) in which the gamma-carboxyl was replaced by a 1H-tetrazol-5-yl ring, an isosteric group with acidic properties similar to a carboxyl group, were investigated. The tetrazolyl analogues of MTX and AMT were more potent inhibitors of the growth of CCRF-CEM and K562 human leukemia cell lines during continuous (120 h) and 24-h pulse exposure than were the respective parent drugs; only when the exposure time was reduced to 6 h were the parent drugs more potent. These inhibitory effects on growth correlated with the onset of and recovery from inhibition of de novo thymidylate biosynthesis. Growth inhibition by the analogues was protectable by leucovorin. MTX-resistant CCRF-CEM sublines with decreased transport or increased dihydrofolate reductase (DHFR) levels were cross-resistant to the analogues. The analogues were as potent as their parent drugs in inhibiting DHFR activity in vitro and at displacing [3H]MTX from intracellular DHFR. Each analogue was more effective than its parent drug at inhibiting uptake of [3H]MTX into CCRF-CEM cells. The tetrazole analogue of AMT was a linear competitive inhibitor (Kis = 50 microM) of CCRF-CEM folylpolyglutamate synthetase, while the tetrazole analogue of MTX, unlike all other inhibitors, was linear noncompetitive (Kis = 51 microM, Kii = 321 microM). The data suggest that, compared with MTX or AMT, the tetrazole substituent, in place of the gamma-carboxyl group, allows more efficient transport into cells via the reduced folate/MTX carrier and the resulting greater uptake of the analogues leads to inhibition of DNA synthesis and cell death at lower extracellular concentrations during long exposures. The mechanism of cell death could involve inhibition at folypolyglutamate synthetase, but DHFR is the primary target. The low potency of the analogues during short exposure is presumably related to the inability to form the poly-gamma-glutamyl metabolites required for intracellular retention.

Effect of Tea Catechins on Folate Analysis in Green Tea by Microbiological Assay.

Green tea is thought to be a primary source of folate in the Japanese diet, based on folate content analyzed by a microbiological assay. Green tea also contains high amount of catechins, in particular, epigallocatechin gallate (EGCg), which was demonstrated to be able to inhibit the digestive enzyme activities and microbial growth in the folate assay. In the present study, we examined whether tea catechins interfered with components of the folate assay for green tea. A marked inhibitory effect of EGCg on microbial growth was observed at an inhibitory concentration of higher than 10 µg/mL. Tea catechins without the galloyl moiety did not show an inhibitory effect. EGCg inhibited the activity of the three enzymes used for assay sample preparation at an inhibitory concentration of higher than 750 µg/mL for α-amylase, 1,000 µg/mL for protease, and 50 µg/mL for conjugase. However, with each step of the assay, the actual concentration of EGCg was decreased to below the inhibitory concentration of each analytical step. Lack of influence of EGCg on green tea folate assay was confirmed by an addition of folate standard in tea infusion. These results suggested that tea catechins have no practical impact on folate analysis in green tea, using the general microbiological assay.

The properties of the secreted gamma-glutamyl hydrolases from H35 hepatoma cells.

gamma-Glutamyl hydrolase has been partially purified and characterized from conditioned culture medium of H35 hepatoma cells. Evidence for heterogeneity of the enzyme is derived from its elution as three distinct peaks of enzymatic activity when the enzyme is purified by TSK-butyl-Sepharose column chromatography. These three enzyme fractions appear to have identical catalytic properties but, as yet, the basis for their resolution is not understood. A rapid, sensitive and simple assay based on reverse-phase HPLC fluorescent detection with pre-column derivatization using o-phthalaldehyde (OPA) was developed to separate OPA-derivatives of poly-gamma-glutamates and glutamic acid. Using this assay and the standard HPLC assay for pteroylpolyglutamates, the enzyme appears to be an endopeptidase with respect to pteroylpenta-gamma-glutamate (PteGlu5), methotrexate penta-gamma-glutamate (4-NH2-10-CH3PteGlu5) and p-aminobenzoyl-penta-gamma-glutamate (pABAGlu5). The initial products are PteGlu1 (or 4-NH2-10-CH3PteGlu1 or pABAGlu1) and intact tetra-gamma-glutamate, which is subsequently degraded to glutamic acid. When penta-gamma-glutamate is the substrate, the cleavage of the gamma-bonds by the enzyme is less ordered, with the early appearance of mono-, di-, tri- and tetraglutamate. Poly-alpha-glutamate is not a substrate nor are pABA-gamma-Glu5 or penta-gamma-glutamate covalently linked to albumin. 4-NH2-10-CH3PteGlu2 or Glu5 bound to dihydrofolate reductase is not a substrate for the enzyme, offering further evidence that protein-associated poly-gamma-glutamates are poor substrates for gamma-glutamyl hydrolase from H35 hepatoma cells.

Human jejunal brush border folate conjugase. Characteristics and inhibition by salicylazosulfapyridine.

Human jejunal brush border folate conjugase (EC 3.4.22.-) was partially purified and characterized. Three drugs known to be associated with clinical folate deficiency were tested for inhibition of the partially purified enzyme. Using jejunal mucosa from obese patients undergoing intestinal bypass surgery, brush border folate conjugase was purified 50-80-fold by centrifugation, Triton X-100 solubilization and DEAE-Sephadex and Sephacryl S-200 chromatography. Using synthetic pteroyldiglutamyl[14C]glutamate as substrate, the enzyme was found to have a pH optimum of 6.5 and an apparent Km of 1.6 micro M. Incubation of the enzyme with synthetic pteroyl[14C]glutamylhexaglutamate resulted in a spectrum of shorter-chain 14C-labeled pteroylglutamates at 60 min. Pteroyl[14C]glutamate was the major product at 120 min, with quantitative recovery of free glutamate in the incubation medium. Salicylazosulfapyridine was a competitive inhibitor of the enzyme (Ki = 0.13 mM), while ethanol, diphenylhydantoin and salicylazosulfapyridine metabolites had no effect. These data suggest that brush border folate conjugase is an exopeptidase which progressively hydrolyzes glutamyl units from pteroylpolyglutamate, leaving pteroylmonoglutamate as the folate form available for intestinal transport. Inhibition of brush border folate conjugase by salicylazosulfapyridine provides a mechanism for folate malabsorption and deficiency in chronic users of this drug.

Inhibition by selected food components of human and porcine intestinal pteroylpolyglutamate hydrolase activity.

Studies were conducted to determine the in vitro effect of selected food components on activity of the brush border membrane pteroylpolyglutamate hydrolase (folate conjugase) of porcine and human intestine. Foods differed widely in their effects although the pattern of the effects on both porcine and human enzymes was similar. Extracts of legumes, tomatoes, and orange juice consistently inhibited the conjugase activity. Citrate was also inhibitory to some extent. In contrast, extracts of cereal grain flours, whole egg, milk, cabbage, cauliflower, and lettuce caused little inhibition. Purified phytohemagglutinins, soybean trypsin inhibitors, and bovine milk folate-binding protein had no effect on the conjugase activity at the concentrations tested. The food substances that inhibited the conjugase activity did not bind the polyglutamyl folate substrate or inhibit intestinal brush border membrane sucrase and alkaline phosphatase. These findings suggest that food composition may influence folate bioavailability by interfering with the intestinal deconjugation of dietary polyglutamyl folates.

Glutamyl hydrolase: properties and pharmacologic impact.

Glutamyl hydrolase cleaves the poly-gamma-glutamate chain folate and antifolate poly-gamma-glutamates. Its cellular location is lysosomal with large amounts of the enzyme constitutively secreted. The highest levels of glutamyl hydrolase mRNA in humans is found in the liver and kidney. Baculovirus-expressed human enzyme has been used to evaluate the method of hydrolysis of methotrexate-gamma-glu4 and MTA-gamma-glu4. In both cases, the substrates are hydrolyzed by removal of the outer two gamma-glutamate linkages, yielding glu and gamma-glu2 as the glutamate products. Cell lines resistant to 5,10-dideazatetrahydrofolate (lometrexol) have sevenfold higher activities of glutamyl hydrolase. These cultures have a 60% to 90% reduced amount of antifolate polygamma-glutamates and 30% reduced folyl poly-gamma-glutamates. These results suggest the possibility of using glutamyl hydrolase to favorably modulate the activity of antifolate therapy.

Update on antifolate drugs targets.

Antifolate drugs are molecules directed to interfere with the folate metabolic pathway at some level. They can be recognized among the first rationally designed compounds applying the principle of structural analogy with the substrate developing the antimetabolite strategy. This strategy has taken advantage of the basic different features of the microbial and human folate metabolism and therefore allows targeting the pathway at different steps generating a specificity tools for Medicinal Chemists. Two main problems are giving renewed importance to such targets and therefore improving the efforts to discover new targets in the folate metabolism area. The first one is the increasing resistance to the present drugs due to different mechanisms such as the enzyme modification and the increased production of enzymes with not well recognized importance. The second one is the development of techniques directed to highlight the interference at genetic level of molecular probes as antifolate drug to develop new enzymes previously unknown. This approach is defined as genetic approach to drug discovery, from gene to drugs. The present article describes the importance in drug design and discovery of some antifolate targets among the best known at the present status of research such as thymidylate synthase (TS), dhydrofolate reductases, (DHFR) serine hydroxymethyltransferase (SHMT), folyilpolyglutamyl synthetase (FPGS), gamma-glutamyl hydrolase (gamma-GH), glycinamide-ribonucleotide transformylase (GARTfase), amino-imidazole-carboxamide-ribonucleotide transformylase (AICARTfase) and Folate transporters. Discovery, known functions, structure/function studies and inhibition will be described.

Novel inhibitors of carboxypeptidase G2 (CPG2): potential use in antibody-directed enzyme prodrug therapy.

The design and synthesis of potent thiocarbamate inhibitors for carboxypeptidase G2 are described. The best thiocarbamate inhibitor N-(p-methoxybenzenethiocarbonyl)amino-L-glutamic acid 6d, chosen for preliminary investigations of in vitro antibody-directed enzyme prodrug therapy (ADEPT), abrogated the cytotoxicity of a combination of A5B7-carboxypeptidase G2 conjugate and prodrug PGP (N-p-{N,N-bis (2-chloroethyl)amino}phenoxycarbonyl-L-glutamate) toward LS174T cells. This is the first report of a small-molecule enzyme inhibitor proposed for use in conjunction with the ADEPT approach.

Methotrexate analogues. 19. Replacement of the glutamate side chain in classical antifolates by L-homocysteic acid and L-cysteic acid: effect on enzyme inhibition and antitumor activity.

Methotrexate (MTX) and aminopterin (AMT) analogues containing L-homocysteic acid or L-cysteic acid in place of L-glutamic acid were synthesized and tested as inhibitors of dihydrofolate reductase from L1210 cells and folyl polyglutamate synthetase from mouse liver. The ID50 against dihydrofolate reductase was comparable for the MTX and AMT analogues (0.04-0.07 microM), whereas the ID50 against folyl polyglutamate synthetase was 3- to 4-fold lower for the AMT analogues (40-60 microM) than for the MTX analogues (100-200 microM). Thus, N10-substitution has a greater effect on binding to folyl polyglutamate synthetase than dihydrofolate reductase. The cytotoxicity of these compounds was assayed in vitro against L1210 cells, and the AMT analogues again proved more potent (ID50 = 0.03-0.05 microM) than the MTX analogues (ID50 = 0.1-0.4 microM). A similarly increased potency was observed for the AMT analogues against L1210 leukemia in vivo. Though differential cell uptake cannot be ruled out as the basis of increased potency, it is possible that part of the activity of the AMT analogues involves interference with the intracellular polyglutamation of reduced folate cofactors, i.e., that they are "self-potentiating antifolates". Of the four compounds reported, the most active was N-(4-amino-4- deoxypteroyl )-L-homocysteic acid, which produced a 138% increase in life span (ILS) in L1210 leukemic mice when given on a modified bid X 10 schedule at a dose of 2 mg/kg. A comparable ILS was obtained with AMT itself at 0.24 mg/kg. Thus, replacement of gamma-CO2H by gamma-SO3H in the side chain does not decrease therapeutic effect. However, a higher dose is required, presumably to offset pharmacological differences reflecting the inability of the sulfonate group to be polyglutamated .

Mechanism-based inhibitors of folylpoly-gamma-glutamate synthetase and gamma-glutamyl hydrolase: control of folylpoly-gamma-glutamate homeostasis as a drug target.

Intracellular folate pools consist primarily of gamma-glutamyl isopeptide conjugates of reduced forms of the vitamin, folic acid. Biosynthesis of these oligomeric isopeptides is catalyzed by the enzyme folylpoly-gamma-glutamate synthetase (FPGS). The highly anionic character of the oligomers renders them unable to cross the cell membrane and, therefore, these forms of reduced folates (and certain antifolates) accumulate in cells to high concentration. gamma-Glutamyl hydrolase (GH) catalyzes the hydrolysis of the oligo-gamma-glutamates derivatives to monoglutamyl forms, which are substrates for the reduced folate carrier and able to exit the cell. This chapter describes research, primarily from our laboratories, on the design, synthesis, and biochemical evaluation of several novel analogues of glutamic acid, gamma-glutamyl peptides, and derivatives of folic acid as well as of antifolate drugs. These include a series of fluoroglutamic acids, fluoroglutamate-containing isopeptides, phosphorus-containing pseudopeptides, and epoxide-containing peptidomimetics. The fluoroglutamic acids and fluoroglutamate-containing folates and antifolates exhibit position-dependent effects on the reactions catalyzed by FPGS and GH, thus providing insight into the catalytic mechanism and control of these enzymes. The phosphinic acid-containing pseudopeptides are the most potent inhibitors of FPGS identified to date, and were designed based on mechanistic enzymology data from our laboratories and others, prior to the publication of any structural information about the targeted enzymes.

gamma-Glutamyl hydrolase, not glutamate carboxypeptidase II, hydrolyzes dietary folate in rat small intestine.

Dietary polyglutamyl folates are hydrolyzed to monoglutamyl folate derivatives prior to intestinal transport. In humans and pigs, the reaction occurs at pH 6.5 at the jejunal brush border membrane by folate hydrolase and is encoded by the glutamate carboxypeptidase II (GCPII) gene. Intracellular folate hydrolase with an optimal pH of 4.5 is encoded by the gamma-glutamyl hydrolase (gamma-GH) gene and predominates in rats. We determined the respective roles of GCPII and gamma-GH in dietary folate hydrolysis in rat small intestine. Duodenal, jejunal, and ileal mucosa, pancreas, and duodenal luminal fluid were collected from 10 Sprague-Dawley rats that had not been food deprived. Folate hydrolase was assayed at pH 4.5 and 6.5 with and without parahydroxymercuribenzoate (pHMB), an inhibitor of intracellular folate hydrolase. Folate hydrolase activity occurred at pH 4.5 in all tissues, was significantly inhibited by the addition of pHMB at both pH 4.5 and 6.5, and was virtually absent from brush border fractions at pH 6.5. The highest activity was in the postprandial duodenal luminal fluid at pH 4.5. Rat-specific primers for GCPII and gamma-GH were used to detect mRNA expression in pancreas, jejunal mucosa, and liver. GCPII expression was detected only in the liver, whereas gamma-GH was expressed in all 3 tissues. These results suggest that the hydrolysis of polyglutamyl folates in rats requires the intracellular folate hydrolase that is expressed by pancreatic gamma-GH, in contrast to GCPII that is expressed in the jejunal mucosal brush border in pigs and humans. gamma-GH folate hydrolase is abundant in rat postprandial pancreatic secretions and appears to hydrolyze dietary folates in the intestinal lumen prior to intestinal absorption.

Inhibition of gamma-glutamyl hydrolases in human cells by 2-mercaptomethylglutaric acid.

Cultured human lymphocytes and fibroblasts accumulate methotrexate during 24 hours and synthesize methotrexate polyglutamates up to the hexaglutamate, with the triglutamate predominating. In the interval after incubation with methotrexate, drug is lost, metabolites are converted to longer chain-lengths, and methotrexate pentaglutamate predominates. 2-Mercaptomethylglutaric acid, an inhibitor of neutral pH gamma-glutamyl hydrolases in vitro, had little effect on polyglutamate synthesis during incubation of the cells with methotrexate, but maintained for 48 hours almost all the methotrexate as the pentaglutamate when added after the removal of the drug. These findings demonstrate that inhibition of gamma-glutamyl hydrolases is an effective approach to alter the distribution of polyglutamate forms of methotrexate in vivo and indicate that enzymatic hydrolysis may contribute to regulation of polyglutamate chain lengths in human cells.

Evaluation of phosphorus-containing inhibitors of gamma-glutamyl hydrolase.

Several putative, phosphorus-containing inhibitors of gamma-glutamyl hydrolase were synthesized and evaluated for inhibitory activity. The phosphonamidoic acids were shown to be weak competitive inhibitors while both a phosphoramidate diester and a phosphonamidate ester were shown to be potent time-dependent inactivators, presumably through irreversible phosphorylation of an active site nucleophile.

Inactivation and clearance of an anti-CEA carboxypeptidase G2 conjugate in blood after localisation in a xenograft model.

Studies with a conjugate of carboxypeptidase G2 (CPG2) and the F(ab')2 fragment of monoclonal anti-CEA antibody, A5B7, have shown specific localisation in a human colon tumour xenograft, LS174T, growing in nude mice. The conjugate reaches a peak concentration in the tumour within 24 h but enzyme activity in blood remains above a critical value for therapeutic purposes for several days. Here we describe a new monoclonal antibody, SB43, raised against CPG2 which is capable of reducing enzyme activity in blood. In vitro studies demonstrated specific binding of SB43 to CPG2 causing inactivation. Moreover, in the nude mouse model SB43 was also capable of inactivating the enzyme in the circulation within minutes of administration. Radiolabelled native SB43 persisted in blood for several days and appreciable non-specific uptake into the xenograft was also observed. Uptake of SB43 by the tumour, with possible inactivation of CPG2 at this site, could be limited by first coupling the antibody to galactose. This ensured recognition and excretion of SB43 and SB43-enzyme complexes via the liver and their rapid removal from the circulation. Galactosylation had no effect on the ability of SB43 to inactivate the enzyme.

Comparison of pteroylpolyglutamate hydrolase (folate conjugase) from porcine and human intestinal brush border membrane.

1. A comparative study of pteroylpolygluatamte hydrolase (folate conjugase) of brush border membrane vesicles from human and porcine intestine was conducted. 2. The enrichment of conjugase activity during membrane isolation was 5-fold greater for the human than the pig. 3. Porcine and human conjugases exhibited similar Km values and could completely hydrolyze pteroyltriglutamate (PteGlu3) to PteGlu1 via an exohydrolytic process. 4. Pteroic acid, PteGlu1 and anionic polysaccharides did not inhibit human or porcine conjugase. 5. Apparent mol. wts for detergent-enzyme complexes were 237,000 (pig) and greater than 500,000 (human). 6. These results indicate similar kinetic properties and mode of action but differences in physical behavior between the intestinal brush border folate conjugases of human and pig.