Metabolic pathways antimetabolites

Antimetabolites interfere with normal metabolic pathways. They can be grouped into folate antagonists and analogues of purine or pyrimidine bases. Their action is limited to the S-phase of the cell cycle and therefore they target a smaller fraction of cells as compared with alkylating agents. 

Antimetabolites are enzyme inhibitors (see p. 96) that selectively block metabolic pathways. The majority of clinically important cytostatic drugs act on nucleotide biosynthesis. Many of these are modified nucleobases or nucleotides that competitively inhibit their target enzymes (see p. 96). Many are also incorporated into the DNA, thereby preventing replication. 

Antimetabolites are compounds that block the normal metabolic pathways operating in cells. They act by either replacing an endogenous compound in the pathway by a compound whose incorporation into the system either results in a product that can no longer play any further part in the pathway or inhibits... 

Chemotherapeutic agents are grouped by cytotoxic mechanism. The alkylating agents, such as cyclophosphamide [50-18-0] and melphalan [148-82-5], interfere with normal cellular activity by alkylation deoxyribonucleic acid (DNA). Antimetabolites, interfering with complex metabolic pathways in the cell, include methotrexate [59-05-2], 5-fluorouracil [51-21-8], and cytosine arabinoside hydrochloride [69-74-9]. Antibiotics such as bleomycin [11056-06-7] and doxorubicin [23214-92-8] have been used, as have the plant alkaloids vincristine [57-22-7] and vinblastine [865-21-4]. 

A. The antimetabolite plays the role of a substrate If the antimetabolite is capable of undergoing the enzyme-catalyzed reaction with the resulting dissociation of the enzyme-antimetabolite complex into (abnormal) produces) and the free enzyme, then it may be considered an abnormal substrate, or substitute metabolite. As such, it will competitively interfere with the transformation of the normal metabolite the extent of such interference depends on the relative affinity of the antimetabolite for the enzyme as well as on the rate of its conversion and subsequent release by the enzyme (i.e., the turn-over rate of the enzyme-antimetabolite complex). In the extreme (but important) case when the affinity is very high and the turnover rate very low, such antimetabolites act, in effect, as potent enzyme inhibitors, rather than as substitute metabolites (see B.iii) below). In the majority of cases, those classical antimetabolites which are capable of undergoing the enzyme-catalyzed reaction, having affinities and conversion rates comparable to those of the corresponding normal metabolites, exert only a partial and temporary inhibition at those steps of the metabolic pathway in which they themselves are metabolized, and therefore, their effective action as metabolic inhibitors will depend on their inhibition of other targets and on subsequent metabolic events (see Section 2.3.). 

However, this does not apply to the special situation when (1) the enzyme is a synthetase which catalyzes the formation of a covalent bond between the metabolite (or antimetabolite) and a second substrate, and (2) the second substrate is available only in a limited amount. In this case, the antimetabolite competes with the metabolite not only for the enzyme but also for the second substrate, with which it will combine covalently to form an inert product. Although this enzyme-mediated reaction of the antimetabolite is reversible by the corresponding metabolite in a competitive manner, due to its potentially crucial metabolic effect, (i.e., the elimination of another, limiting metabolite which is required for the same reaction step of the metabolic pathway), this reaction per se could be responsible for the over-all inhibitory effect of the antimetabolite. That is, in such particular cases, the metabolic target of the inhibitory action of the antimetabolite may be an enzymic reaction step in which it actually plays the role of a substrate. One might think that this type of situation is a rather special and unusual one, as it may be indeed however, it so happens that the first descovered and still important class of classical and semi-classical antimetabolites, the sulfonamides, appears to act in this manner, as indicated by the results of a recent study8 (see Section 3.2.). 

A. Metabolic activation The antimetabolites (and their subsequent enzymic reaction product(s), respectively) may be utilized as competitive substrate(s) in one (or several consecutive) enzymic reaction(s) along the metabolic pathway of the normal metabolite, but at one stage of the metabolic reaction sequence, the transformed analogue cannot be further utilized as a substrate and, instead, acts as an inhibitor of the enzyme which catalyzes the next reaction step. At this stage, the action of the transformed ( activated ) analogue as an enzyme inhibitor depends on the same general types of structural requirements as outlined in the case of the directly acting enzyme inhibitors (see Section 2.2. ... 

Antimetabolites block or alter a metabolic pathway involved in DNA synthesis. They are analogues or antagonists of normal cell constituents. 

The exploration of enzymes and metabolic pathways by means of antimetabolites has been productive of both new medicinal agents and advances in fundamental knowledge. ... 

MVA is now known to be metabolized by routes other than those which give rise to terpenoids and steroids. The breakdown occurs predominantly in the kidneys to give C2 units that can be utilized in fatty-acid synthesis. The sterol and the shunt pathways have been evaluated in nine different tissues of rat previous conclusions that the kidneys are the predominant site of both types of metabolism have been confirmed. MVA is known to accumulate, at a low level, in the blood, and these results suggest that impairment of renal clearance of serum MVA by either route may account for the hypercholesterolaemia associated with some kidney disorders. A study of the effects of possible antimetabolites of MVA (for example the 2,3-anhydro-compound) on the formation of cholesterol in cell-free systems from liver has been reported. ... 

Metabolic inhibitors may fail even where no species selectivity is required. The antimetabolite A4)5-cholestenone, designed to inhibit cholesterol synthesis, illustrates this it blocks the conversion of desmosterol to cholesterol, the final step in this pathway (Fig. 4). The blockade of cholesterol formation, however,... 

An interesting example of a DDI due to the inhibition of a non-CYP enzyme that can have serious clinical consequences is the inhibition of xanthine oxidase by allopu-rinol 6-mercaptopurine (6-MP) as an antimetabolite type of antineoplastic drug. One of its indications is in the treatment of inflammatory bowel disease. Actually, 6-MP is a prodrug whose active metabolite, 6-thiogua-nine (6-TG) is responsible for its therapeutic activity. Some nonresponders to 6-MP do not form sufficient amounts of 6-TG. A complementary pathway of 6-MP metabolism is oxidation to 6-thiouric acid (6TU), which is mediated by xanthine oxidase. Inhibition of this complementary pathway by allopurinol shunts the metabolism of 6-MP favoring increased formation of 6-TG. 

Fluorouracil is a pyrimidine antimetabolite. The metabolism of fluorouracil in the anabolic pathway blocks the methylation reaction of deoxyuridylic acid to chymidylic acid. In this manner, fluorouracil interferes with the synthesis of DNA and to a lesser extent inhibits the formation of RNA. It is indicated in colon, rectum, breast, gastric, and pancreatic carcinoma (injection) multiple actinic or solar keratoses, and superficial basal-cell carcinoma (topical). 

Metabolism For uptake and transport of G., see Lit. d-G. plays a central part in the carbohydrate metabolism. It is degraded to smaller molecules in complicated reaction sequences (glycolysis) with release of energy - one example is pyruvic acid, which can enter the citric acid cycle via acetyl-CoA - or (pentose phosphate pathway) can be converted to derivatives of other sugars for biosynthetic purposes under the concomitant availability of reduction equivalents. Alternatively d-G. can be stored in the liver and muscles as areserve substance glycogen (in plants starch). An antimetabolite of d-G. is 5-thio-D-glucose. For detection, see Lit.. ... 

Although there is little doubt that de novo synthesis provides most of the purine nucleotides used in metabolism, alternative pathways have been described. They assume particular importance because they can be used to bypass metabolic blocks caused by antimetabolites. 

With aminopterin, the reductase forms a complex with a low dissociation constant. Thus, in the presence of aminopterin many of the reductase molecules are trapped in an inactive form. The coenzymes necessary for purine biosynthesis are not formed, and that pathway is blocked. In this manner aminopterin interferes with the progress of leukemia and with the proliferation of normal bone marrow. Unfortunately, the cells of individuals treated for leukemia overcome the metabolic block by building up a resistance to the antimetabolites by increasing the level of the reductase. We will now consider the mechanism of action of each of these coenzymes separately. 

For the past decade this laboratory has devoted much of its attention to an examination of various facets of purine metabolism in human erythrocytes. These cells do not have the complete pathway for the novo synthesis of purines and do not make nucleic acids. On the other hand, they have an active nucleotide metabolism and contain the salvage enzymes, hypoxan-thine-guanine phosphoribosyl transferase (HGPRTase), adenine phosphoribosyl transferase (APRTase) and adenosine kinase. In view of the fact that the activities of certain enzymes of purine metabolism are quite high (e.g., purine nucleoside phos-phorylase occurs at a level of about 15 umolar units/ml of erythrocytes) and the total mass of erythrocytes in the adult human being is in excess of two liters, it appears that these cells play an important and perhaps not yet fully appreciated role in the whole body economy of purines in man. Therefore, we believe that the human erythrocyte provides a very useful model system for the examination of purine metabolism in man as well as for investigations of the action of certain purine and purine nucleoside antimetabolites, many of which are important in medicine. 

---