Selective inhibition of human Molt-4 leukemia type II inosine 5'-monophosphate dehydrogenase by the 1,5-diazabicyclo[3.1.0]hexane-2,4-diones

Role of salt bridge dynamics in inter domain recognition of human IMPDH isoforms: an insight to inhibitor topology for isoform-II

Inosine monophosphate dehydrogenase (IMPDH) enzyme involves in the biosynthesis pathway of guanosine nucleotide. Type II isoform of the enzyme is selectively upregulated in neoplastic fast replicating lymphocytes and CML cancer cells. The hIMPDH-II is an excellent target for antileukemic agent. The detailed investigation during MD-Simulation (15 ns) of three different unliganded structures (1B3O, 1JCN and 1JR1) have clearly explored the salt bridge mediated stabilization of inter or intra domain (catalytic domains I(N), I(C) with res. Id. 28-111 and 233-504, whereas two CBS domains C₁, C₂ are 112-171 and 172-232) in IMPDH enzyme which are mostly inaccessible in their X-rays structures. The salt bridge interaction in I(N)---C₁ inter-domain of hIMPDH-I, I(N)---C₂ of IMPDH-II and C₁---I(C) of nhIMPDH-II are discriminative features among the isoforms. The I(N)---C₂ recognition in hIMPDH-II (1B3O) is missing in type-I isoform (1JCN). The salt bridge interaction D232---K238 at the surface of protein and the involvement of three conserved water molecules or the hydrophilic centers (WA²³²(OD1), WB ²³²(OD2) and W²³⁸(NZ)) to those acidic and basic residues seem to be unique in hIMPDH-II. The hydrophilic susceptibility, geometrical and electronic consequences of this salt bridge interaction could be useful to design the topology of specific inhibitor for hIMPDH-II which may not be effective for hIMPDH-I. Possibly, the aliphatic ligand containing carboxyl, amide or hydrophilic groups with flexible structure may be implicated for hIMPDH-II inhibitor design using the conserved water mimic drug design protocol.

Inosine monophosphate dehydrogenase as a target for antiviral, anticancer, antimicrobial and immunosuppressive therapeutics

Inosine monophosphate dehydrogenase (IMPDH) catalyzes the rate-limiting step in the de novo biosynthesis of guanine nucleotides. In recent years it has become the target of multiple drugs in an attempt to cure a variety of diseases. Possible therapeutic drugs range from antiviral and anticancer to immunosuppressive targets. Research has shown that if IMPDH is effectively inhibited, cancerous growth can be slowed and virus replication can be stopped. Microbial and parasitic IMPDH differ significantly from the human isoforms and targeting those isoforms could lead to effective treatments for many diseases. Inhibiting IMPDH is an extremely promising therapy for a variety of disease states. Isoform- and species-selective inhibition is desirable and scientists are making significant progress in these areas.

Inosine monophosphate dehydrogenase (IMPDH) as a target in drug discovery

Inosine monophosphate dehydrogenase (IMPDH) is a key enzyme of de novo purine nucleotide biosynthesis and is viewed as an important target in the quest for discovery of drugs in the antiviral, antibacterial and anticancer therapeutic areas. This review focuses on the medicinal chemistry, drug discovery and chemical biology of IMPDH. Examples of IMP and cofactor site-directed inhibitors, allosteric inhibitors and isoform-selective inhibitors are presented. Comparison of IMPDHs from different organisms is also made to facilitate the design of species-selective IMPDH inhibitors for drug discovery. Special emphasis in the review is placed on IMPDH from Mycobacterium tuberculosis.

Mechanism of action of the antitumor agents 6-benzoyl-3,3-disubstituted-1,5-diazabicyclo[3.1.0]hexane-2,4-diones: potent inhibitors of human type II inosine 5'-monophosphate dehydrogenase

The observation that expression of the IMPDH gene is tightly linked with cellular proliferation and transformation has led to an interest in developing inhibitors that deplete intracellular guanine nucleotide pools. IMPDH exists as 2 isoforms, one of which is induced in tumor cells, type II and thus has led to new interest in this target for the design of isoform-selective anticancer chemotherapeutic agents. Several classes of IMPDH inhibitor are now in use or under development; however, only the 1,5-diazabicyclo[3.1.0]hexane-2,4-diones show selectivity for the type II isoform. In the current study, we further evaluated chemical modification of this class to determine the necessary components for selective type II IMPDH inhibition. The 6-benzoyl-3,3-disubstituted-1,5-diazabicyclo[3.1.0]hexane-2,4-diones were effective cytotoxic agents in human leukemias, lymphomas, breast, glioma and HeLa-S3 suspended uterine carcinoma screens with ED(50) values 0.3 to 12 microM. The agents acted as antimetabolites by inhibiting de novo purine biosynthesis at the key regulatory enzyme IMPDH, resulting in suppression of DNA synthesis and dGTP pool levels within 60 min. Furthermore, the derivatives were specific for the type II isoform as opposed to type I, acting in a competitive manner with K(i) values of 5.1 to 63 microM. Addition of the 6-benzoyl moiety to the bicyclic parent ring structure afforded the most potent agent in the novel class of 1,5-diazabicyclo[3.1.0]hexane-2,4-diones that selectively inhibits type II IMPDH activity.

Specific inhibition of type II inosine monophosphate dehydrogenase activity of Tmolt4 T cell human leukaemia cells by 3-methoxy and di-benzohydroxamic acids, maleic hydrazide and malonic acids

Small-molecular-weight benzohydroxamic and malonic acids and maleic hydrazide proved to be potent inhibitors of the activity of human Tmolt4 leukaemia Type II IMP (inosine monophosphate) dehydrogenase (IMPDH) activity. They were competitive inhibitors with respect to IMPDH demonstrating Ki values in the range 2.57-41.3 microM, less than half the values of the IC50 (microM) for the inhibition of Type II IMPDH. The IC50 microM values positively correlated with the ability of each compound to inhibit crude IMPDH activity, de-novo purine and DNA syntheses and growth of the T leukaemia cell line. Compounds were not inhibitors of Type I IMPDH. Type I IMPDH predominates in normal resting cells compared with Type II which is found in rapidly proliferating cells. Discovery of agents which would selectivity target IMPDH found in proliferating cells should eliminate any antineoplastic therapeutic toxic effects in normal cells of the body.

Targeting of human Tmolt4 leukemic type II IMP dehydrogenase by cyclic imide related derivatives

2,3-Dihydrophthalazine-1,4-diones, indazolones, 3-imino-1-oxoisodolines, homophthalimides, napthalidimides, diphenamides, and 6,7-dihydro-5H-dibenz[c,e]azepines proved to be potent inhibitors of the activity of human Tmolt4 T cell leukemia Type II IMP dehydrogenase (IMPDH). This inhibition was competitive, yielding Ki values in the range of 1.96 to 48.9 microM. The inhibition of Type II IMPDH correlated positively with the inhibition of the growth of Tmolt4 cells, the syntheses of DNA and purine, and the activity of crude IMPDH. The Type II IMPDH isoform is found in rapidly proliferating cells. The isoform present in normal resting cells, Type I IMPDH, was elevated by the compounds at 100 microM. In addition, Compound 5 significantly increased the Type I enzyme activity in a concentration and time dependent manner. The selectivity of these derivatives towards Type II IMPDH will allow for the separation of cellular effects, which should reduce clinical toxicity when treating with antimetabolite IMPDH inhibitors.

Implications of selective type II IMP dehydrogenase (IMPDH) inhibition by the 6-ethoxycarbonyl-3,3-disubstituted-1,5-diazabicyclo[3.1.0]hexane-2,4-diones on tumor cell death

It was shown previously that three 1,5-diazabicyclo[3.1.0]hexane-2,4-diones selectively inhibited human Type II IMP dehydrogenase (IMPDH) from Tmolt4 cell leukemia [Barnes et al., Biochemistry 2000;39:13641-50]. The agents acted as competitive inhibitors of this isoform, yet when tested against human Type I at concentrations ranging from 0.5 to 500 microM, Type I was not inhibited. This study focuses on the antineoplastic activity and cellular effects of one of these agents and two new derivatives containing ethoxycarbonyl substitution at position C6. Agents were studied for antiproliferative activity in human Tmolt4 leukemia (EC(50) 3.3 to 9.2 microM) and alterations in the levels of enzymes involved with cellular metabolism, including DNA and RNA syntheses due to IMPDH inhibition. Results reported here demonstrate that 6-ethoxycarbonyl-3,3-disubstituted-1,5-diazabicyclo[3.1.0]hexane-2,4-diones are effective inhibitors of DNA synthesis (30-66% inhibition) due to reductions in dGTP pool levels. Collectively, the three agents proved to be selective inhibitors of human IMPDH Type II activity (K(i) 11-33 microM), leading to cytotoxicity in a number of suspended and solid tumor lines, notably MCF-7 (EC(50) 0.7 to 6.0 microM). In addition, negative cytotoxic actions of these agents on WI-38 cell growth, a normal rapidly growing human line, suggest that specific targeting of Type II IMPDH would help to eliminate cell killing in lines where Type I predominates. Furthermore, effects of agents on DNA synthesis and cell death could be reversed by the addition of exogenous guanosine to the medium. Results from in vitro studies suggest that the 6-ethoxycarbonyl-3,3-disubstituted-1,5-diazabicyclo[3.1.0]hexane-2,4-diones may be used as effective isozyme-selective chemotherapeutic agents.

Tmolt4 leukemic type II isoform of IMP dehydrogenase as a target for 1,2,4-triazolidine-3,5-diones, 1-(1-(3-methylphenyl)ethylidineamino)-4,4-diethyl-3,5-azetidinediones, 3,5-isoxazolidinediones, and 4,4-disubstituted-3,5-pyrazolidinediones

The 1,2,4-triazolidine-3,5-diones, 1-(1-(3-methylphenyl)ethylidineamino)-4,4-diethyl-3,5-azetidinediones, and 4,4-disubstituted-3,5-pyrazolidinediones proved to be potent competitive inhibitors of human Tmolt4 leukemia Type II IMP dehydrogenase [IMPDH] activity, an enzyme isoform which is induced in highly proliferating cells. On the other hand, the 3,5-isoxazolidinediones were shown to be uncompetitive inhibitors of Type II IMPDH activity. The correlation between inhibition of Type II IMPDH activity with the agents' ability to suppress DNA and purine syntheses in these Tmolt4 leukemia cell was positive. Type I IMPDH (i.e., the isoform that is present in normal cells) was not inhibited by these compounds suggesting that these agents would be less toxic to normal cells and have selective inhibition towards proliferating cells.