A Structural Basis for Biguanide Activity

Insights into the action of the pharmaceutical metformin: Targeted inhibition of the gut microbial enzyme agmatinase

Metformin is the first-line treatment for type 2 diabetes, yet its mechanism of action is not fully understood. Recent studies suggest metformin's interactions with gut microbiota are responsible for exerting therapeutic effects. In this study, we report that metformin targets the gut microbial enzyme agmatinase, as a competitive inhibitor, which may impair gut agmatine catabolism. The metformin inhibition constant (Ki) of E. coli agmatinase is 1 mM and relevant in the gut where the drug concentration is 1-10 mM. Metformin analogs phenformin, buformin, and galegine are even more potent inhibitors of E. coli agmatinase (Ki = 0.6, 0.1, and 0.007 mM, respectively) suggesting a shared mechanism. Agmatine is a known effector of human host metabolism and has been reported to augment metformin's therapeutic effects for type 2 diabetes. This gut-derived inhibition mechanism gives new insights on metformin's action in the gut and may lead to significant discoveries in improving metformin therapy.

Glucose-Responsive Microparticle-Loaded Dissolving Microneedles for Selective Delivery of Metformin: A Proof-of-Concept Study

Diabetes mellitus (DM) is a metabolic disorder that is one of the most common health problems in the world, primarily type 2 DM (T2DM). Metformin (MTF), as the first-line treatment of DMT2, is effective in lowering glucose levels, but its oral administration causes problems, including gastrointestinal side effects, low bioavailability, and the risk of hypoglycemia. In this study, we formulated MTF into microparticles incorporating a glucose-responsive polymer (MP-MTF-GR), which could potentially increase the bioavailability and extend and control the release of MTF according to glucose levels. This system was delivered by dissolving microneedles (MP-MTF-GR-DMN), applied through the skin, thereby preventing gastrointestinal side effects of orally administered MTF. MP-MTF-GR was formulated using various concentrations of gelatin as a polymer combined with phenylboronic acid (PBA) as a glucose-responsive material. MP-MTF-GR was encapsulated in DMN using polyvinyl pyrrolidone (PVP) and polyvinyl alcohol (PVA) as DMN polymers. The physicochemical evaluation of MP-MTF-GR showed that MTF could be completely entrapped in MP with the percentage of MTF trapped increasing with increasing gelatin concentration without changing the chemical structure of MTF and producing stable MP. In addition, the results of the physicochemical evaluation of MP-MTF-GR-DMN showed that DMN had adequate mechanical strength properties and penetration ability and was stable to environmental changes. The results of the in vitro release and ex vivo permeation study on media with various concentrations of glucose showed that the release and permeation of MTF from the formula increased with increasing glucose levels in the media. The MP-MTF-GR-DMN formula successfully delivered MTF through the skin at 11.30 ± 0.29, 23.31 ± 1.64, 36.12 ± 3.77, and 53.09 ± 3.01 μg from PBS, PBS + glucose 1%, PBS + glucose 2%, and PBS + glucose 4%, respectively, at 24 h, which indicates glucose-responsive permeation and release behavior. The formula developed was also proven to be nontoxic based on hemolysis tests. Importantly, the in vivo study on the rat model showed that this combination approach could provide a better glucose reduction compared to other routes, reducing the blood glucose level to normal levels after 3 h and maintaining this level for 8 h. Furthermore, this approach did not change the skin moisture of the rats. This MP-MTF-GR-DMN is a promising alternative to MTF delivery to overcome MTF problems and increase the effectiveness of T2DM therapy.

Dihydrofolate reductase, thymidylate synthase, and serine hydroxy methyltransferase: successful targets against some infectious diseases

Parasitic diseases have a serious impact on the world in terms of health and economics and are responsible for worldwide mortality and morbidity. The present review features the hybrid targeting involving three main enzymes for the treatment of different parasitic diseases. The enzymes Dihydrofolate reductase, thymidylate synthase, and Serine hydroxy methyltransferase play an essential role in the folate pathway. The present review focuses on these enzymes, which can be targeted against several diseases. It shed light on the past, present, and future of these targets, and it can be assessed that these targets can play a significant role against several infectious diseases. For combating viral and protozoal infectious diseases, these targets in combination should be addressed.

Chloride intracellular channel 1 activity is not required for glioblastoma development but its inhibition dictates glioma stem cell responsivity to novel biguanide derivatives

Background

Chloride intracellular channel-1 (CLIC1) activity controls glioblastoma proliferation. Metformin exerts antitumor effects in glioblastoma stem cells (GSCs) inhibiting CLIC1 activity, but its low potency hampers its translation in clinical settings.

Methods

We synthesized a small library of novel biguanide-based compounds that were tested as antiproliferative agents for GSCs derived from human glioblastomas, in vitro using 2D and 3D cultures and in vivo in the zebrafish model. Compounds were compared to metformin for both potency and efficacy in the inhibition of GSC proliferation in vitro (MTT, Trypan blue exclusion assays, and EdU labeling) and in vivo (zebrafish model), migration (Boyden chamber assay), invasiveness (Matrigel invasion assay), self-renewal (spherogenesis assay), and CLIC1 activity (electrophysiology recordings), as well as for the absence of off-target toxicity (effects on normal stem cells and toxicity for zebrafish and chick embryos).

Results

We identified Q48 and Q54 as two novel CLIC1 blockers, characterized by higher antiproliferative potency than metformin in vitro, in both GSC 2D cultures and 3D spheroids. Q48 and Q54 also impaired GSC self-renewal, migration and invasion, and displayed low systemic in vivo toxicity. Q54 reduced in vivo proliferation of GSCs xenotransplanted in zebrafish hindbrain. Target specificity was confirmed by recombinant CLIC1 binding experiments using microscale thermophoresis approach. Finally, we characterized GSCs from GBMs spontaneously expressing low CLIC1 protein, demonstrating their ability to grow in vivo and to retain stem-like phenotype and functional features in vitro. In these GSCs, Q48 and Q54 displayed reduced potency and efficacy as antiproliferative agents as compared to high CLIC1-expressing tumors. However, in 3D cultures, metformin and Q48 (but not Q54) inhibited proliferation, which was dependent on the inhibition dihydrofolate reductase activity.

Conclusions

These data highlight that, while CLIC1 is dispensable for the development of a subset of glioblastomas, it acts as a booster of proliferation in the majority of these tumors and its functional expression is required for biguanide antitumor class-effects. In particular, the biguanide-based derivatives Q48 and Q54, represent the leads to develop novel compounds endowed with better pharmacological profiles than metformin, to act as CLIC1-blockers for the treatment of CLIC1-expressing glioblastomas, in a precision medicine approach.

Metal Complexes—A Promising Approach to Target Biofilm Associated Infections

Microbial biofilms are represented by sessile microbial communities with modified gene expression and phenotype, adhered to a surface and embedded in a matrix of self-produced extracellular polymeric substances (EPS). Microbial biofilms can develop on both prosthetic devices and tissues, generating chronic and persistent infections that cannot be eradicated with classical organic-based antimicrobials, because of their increased tolerance to antimicrobials and the host immune system. Several complexes based mostly on 3D ions have shown promising potential for fighting biofilm-associated infections, due to their large spectrum antimicrobial and anti-biofilm activity. The literature usually reports species containing Mn(II), Ni(II), Co(II), Cu(II) or Zn(II) and a large variety of multidentate ligands with chelating properties such as antibiotics, Schiff bases, biguanides, N-based macrocyclic and fused rings derivatives. This review presents the progress in the development of such species and their anti-biofilm activity, as well as the contribution of biomaterials science to incorporate these complexes in composite platforms for reducing the negative impact of medical biofilms.

Synthesis, Anti-Proliferative Evaluation and Mechanism of 4-Trifluoro Methoxy Proguanil Derivatives with Various Carbon Chain Length

Among the known biguanide drugs, proguanil has the best antiproliferative activity. In contrast, newly synthesized biguanide derivatives containing fluorine atoms have excellent biological activity, among which trifluoromethoxy compounds show the strongest ability. Preliminary work in our laboratory exhibited that n-heptyl containing proguanil derivatives on one alkyl chain side have better biological activity than those with a shorter carbon chain. However, the relationship between the length of the carbon chain and the activity of the compounds is unknown. In this study, we synthesized 10 new trifluoromethoxy-containing proguanil derivatives with various carbon chain lengths. The phenyl side is fixed as the trifluoromethoxy group with change of carbon chain length in alkyl chain side. It was found that the anti-cancer abilities of 5C-8C with n-pentyl to n-octyl groups was significantly better than that of proguanil in the five human cancer cell lines. The colony formation assay demonstrated that 6C-8C at 0.5 to 1.0 μM significantly inhibited the colony formation of human cancer cell lines, much stronger than that of proguanil. Pharmacologically, 8C activates AMPK, leading to inactivation of the mTOR/p70S6K/4EBP1 pathway. Thus, these novel compounds have a great potential for developing new anti-cancer candidates.

Metformin Is a Pyridoxal-5'-phosphate (PLP)-Competitive Inhibitor of SHMT2

Simple Summary

The mitochondrial enzyme serine hydroxymethyltransferase (SHMT2), which converts serine into glycine and generates 1C units for cell growth, is one of the most consistently overexpressed metabolic enzymes in cancer. Here, we reveal that the anti-diabetic biguanide metformin operates as a novel class of non-catalytic SHMT2 inhibitor that disrupts the pyridoxal-5′-phosphate (PLP)-dependent SHMT2 oligomerization process and ultimately SHMT2 activity. As SHMT2 inhibitors have not yet reached the clinic, these findings may aid the rational design of PLP-competitive SHMT2 inhibitors based on the biguanide skeleton of metformin.

Abstract

The anticancer actions of the biguanide metformin involve the functioning of the serine/glycine one-carbon metabolic network. We report that metformin directly and specifically targets the enzymatic activity of mitochondrial serine hydroxymethyltransferase (SHMT2). In vitro competitive binding assays with human recombinant SHMT1 and SHMT2 isoforms revealed that metformin preferentially inhibits SHMT2 activity by a non-catalytic mechanism. Computational docking coupled with molecular dynamics simulation predicted that metformin could occupy the cofactor pyridoxal-5′-phosphate (PLP) cavity and destabilize the formation of catalytically active SHMT2 oligomers. Differential scanning fluorimetry-based biophysical screening confirmed that metformin diminishes the capacity of PLP to promote the conversion of SHMT2 from an inactive, open state to a highly ordered, catalytically competent closed state. CRISPR/Cas9-based disruption of SHMT2, but not of SHMT1, prevented metformin from inhibiting total SHMT activity in cancer cell lines. Isotope tracing studies in SHMT1 knock-out cells confirmed that metformin decreased the SHMT2-channeled serine-to-formate flux and restricted the formate utilization in thymidylate synthesis upon overexpression of the metformin-unresponsive yeast equivalent of mitochondrial complex I (mCI). While maintaining its capacity to inhibit mitochondrial oxidative phosphorylation, metformin lost its cytotoxic and antiproliferative activity in SHMT2-null cancer cells unable to produce energy-rich NADH or FADH₂ molecules from tricarboxylic acid cycle (TCA) metabolites. As currently available SHMT2 inhibitors have not yet reached the clinic, our current data establishing the structural and mechanistic bases of metformin as a small-molecule, PLP-competitive inhibitor of the SHMT2 activating oligomerization should benefit future discovery of biguanide skeleton-based novel SHMT2 inhibitors in cancer prevention and treatment.

Influence of solution ionic strength on the stabilities of M20 loop conformations in apo E. coli dihydrofolate reductase

Interactions among ions and their specific interactions with macromolecular solutes are known to play a central role in biomolecular stability. However, similar effects in the conformational stability of protein loops that play functional roles, such as binding ligands, proteins, and DNA/RNA molecules, remain relatively unexplored. A well-characterized enzyme that has such a functional loop is Escherichia coli dihydrofolate reductase (ecDHFR), whose so-called M20 loop has been observed in three ordered conformations in crystal structures. To explore how solution ionic strengths may affect the M20 loop conformation, we proposed a reaction coordinate that could quantitatively describe the loop conformation and used it to classify the loop conformations in representative ecDHFR x-ray structures crystallized in varying ionic strengths. The Protein Data Bank survey indicates that at ionic strengths (I) below the intracellular ion concentration-derived ionic strength in E. coli (I ≤ 0.237M), the ecDHFR M20 loop tends to adopt open/closed conformations, and rarely an occluded loop state, but when I is >0.237M, the loop tends to adopt closed/occluded conformations. Distance-dependent electrostatic potentials around the most mobile M20 loop region from molecular dynamics simulations of ecDHFR in equilibrated CaCl₂ solutions of varying ionic strengths show that high ionic strengths (I = 0.75/1.5M) can preferentially stabilize the loop in closed/occluded conformations. These results nicely correlate with conformations derived from ecDHFR structures crystallized in varying ionic strengths. Altogether, our results suggest caution in linking M20 loop conformations derived from crystal structures solved at ionic strengths beyond that tolerated by E. coli to the ecDHFR function.

Biguanides: Species with versatile therapeutic applications

Biguanides are compounds in which two guanidine moieties are fused to form a highly conjugated system. Biguanides are highly basic and hence they are available as salts mostly hydrochloride salts, these cationic species have been found to exhibit many therapeutic properties. This review covers the research and development carried out on biguanides and accounts the various therapeutic applications of drugs containing biguanide group-such as antimalarial, antidiabetic, antiviral, anticancer, antibacterial, antifungal, anti-tubercular, antifilarial, anti-HIV, as well as other biological activities. The aim of this review is to compile all the medicinal chemistry applications of this class of compounds so as to pave way for the accelerated efforts in finding the drug action mechanisms associated with this class of compounds. Importance has been given to the organic chemistry of these biguanide derivatives also.

Hydrochlorothiazide and Indapamide bind the NADPH binding site of bacterial Dihydrofolate Reductase: results of an in-silico study and their implications

Hypertension is a public health concern. Low dose thiazide diuretics are known to effectively control blood pressure compared to that of other classes of antihypertensive drugs. In this context, we have performed an in-silico study and found that the two Sulphonamide Diuretics Hydrochlorothiazide and Indapamide bound the NADPH binding region of bacterial Dihydrofolate Reductase. Therefore, akin to Sulphonamide Antibiotics, Sulphonamide Diuretics may have antibiotic activity and thereby have the potential to modulate the gut microbiome in a way beneficial to vascular health. The in-silico experiment results were analyzed in the context of the relevant literature. We postulate that Sulphonamide Diuretics exert their antihypertensive role by modulating the gut microbiome, specifically by increasing butyrate-producing taxa in the gut. We recommend extending such work as it is plausible that Indapamide and other Sulphonamide Diuretics may be beneficial for both diabetes and hypertension.

The Structural Basis for Nonsteroidal Anti-Inflammatory Drug Inhibition of Human Dihydrofolate Reductase

Although nonsteroidal anti-inflammatory drugs (NSAIDs) target primarily cyclooxygenase enzymes, a subset of NSAIDs containing carboxylate groups also has been reported to competitively inhibit dihydrofolate reductase (DHFR). In this study, we have characterized NSAID interactions with human DHFR based on kinetic, NMR, and X-ray crystallographic methods. The NSAIDs target a region of the folate binding site that interacts with the p-aminobenzoyl-l-glutamate (pABG) moiety of folate and inhibit cooperatively with ligands that target the adjacent pteridine-recognition subsite. NSAIDs containing benzoate or salicylate groups were identified as having the highest potency. Among those tested, diflunisal, a salicylate derivative not previously identified to have anti-folate activity, was found to have a K_i of 34 μM, well below peak plasma diflunisal levels reached at typical dosage levels. The potential of these drugs to interfere with the inflammatory process by multiple pathways introduces the possibility of further optimization to design dual-targeted analogs.

Metformin induces a fasting- and antifolate-mimicking modification of systemic host metabolism in breast cancer patients

Certain dietary interventions might improve the therapeutic index of cancer treatments. An alternative to the “drug plus diet” approach is the pharmacological reproduction of the metabolic traits of such diets. Here we explored the impact of adding metformin to an established therapeutic regimen on the systemic host metabolism of cancer patients. A panel of 11 serum metabolites including markers of mitochondrial function and intermediates/products of folate-dependent one-carbon metabolism were measured in paired baseline and post-treatment sera obtained from HER2-positive breast cancer patients randomized to receive either metformin combined with neoadjuvant chemotherapy and trastuzumab or an equivalent regimen without metformin. Metabolite profiles revealed a significant increase of the ketone body β-hydroxybutyrate and of the TCA intermediate α-ketoglutarate in the metformin-containing arm. A significant relationship was found between the follow-up levels of homocysteine and the ability of treatment arms to achieve a pathological complete response (pCR). In the metformin-containing arm, patients with significant elevations of homocysteine tended to have a higher probability of pCR. The addition of metformin to an established anti-cancer therapeutic regimen causes a fasting-mimicking modification of systemic host metabolism. Circulating homocysteine could be explored as a clinical pharmacodynamic biomarker linking the antifolate-like activity of metformin and biological tumor response.

Pharmacology of metformin - An update

Despite being a successful diabetes type 2 drug for more than a half-century in Europe, the mode of action of metformin is still debated. It is the purpose of this review to inform the reader about most recent findings for metformin with respect to its antidiabetic activity as well as proposed benefits beyond glucose control in humans. Clinical evidence now suggests that most of metformin benefits originate from its actions in the gut, involving hormone signaling by glucagon-like peptide 1 and peptide YY. Growth differentiation factor 15, also mainly produced in the gut, was first identified as a biomarker for metformin use but is now suggested to play a significant role in e.g. weight loss of prediabetics. The pharmacokinetics of the drug in humans as basis for pharmacodynamics, resulting in high tissue levels of the intestinal wall, including the colon, proven by biopsies, is presented. A critical survey of metformin actions on mitochondria, increasing the AMP/ATP ratio but also acting as a mild uncoupler, and of postulated new cellular targets (lysosomes) is included.

The crystal structure of a tetrahydrofolate-bound dihydrofolate reductase reveals the origin of slow product release

Dihydrofolate reductase (DHFR) catalyzes the stereospecific reduction of 7,8-dihydrofolate (FH2) to (6s)-5,6,7,8-tetrahydrofolate (FH4) via hydride transfer from NADPH. The consensus Escherichia coli DHFR mechanism involves conformational changes between closed and occluded states occurring during the rate-limiting product release step. Although the Protein Data Bank (PDB) contains over 250 DHFR structures, the FH4 complex structure responsible for rate-limiting product release is unknown. We report to our knowledge the first crystal structure of an E. coli. DHFR:FH4 complex at 1.03 Å resolution showing distinct stabilizing interactions absent in FH2 or related (6R)-5,10-dideaza-FH4 complexes. We discover the time course of decay of the co-purified endogenous FH4 during crystal growth, with conversion from FH4 to FH2 occurring in 2–3 days. We also determine another occluded complex structure of E. coli DHFR with a slow-onset nanomolar inhibitor that contrasts with the methotrexate complex, suggesting a plausible strategy for designing DHFR antibiotics by targeting FH4 product conformations.

Hongnan Cao et al. present the X-ray crystal structure of E. coli dihydrofolate reductase (DHFR) in complex with its reduced substrate, (6s)-5,6,7,8-tetrahydrofolate (FH4). This structure provides the first glimpse of the rate-limiting product release step of the DHFR mechanism and suggests a strategy for designing DHFR-targeting antibiotics.

Effects of Antidiabetic Drugs on Gut Microbiota Composition

Gut microbiota forms a catalog of about 1000 bacterial species; which mainly belong to the Firmicutes and Bacteroidetes phyla. Microbial genes are essential for key metabolic processes; such as the biosynthesis of short-chain fatty acids (SCFA); amino acids; bile acids or vitamins. It is becoming clear that gut microbiota is playing a prevalent role in pathologies such as metabolic syndrome; type 2 diabetes (T2D); inflammatory and bowel diseases. Obesity and related diseases; notably type 2 diabetes, induce gut dysbiosis. In this review; we aim to cover the current knowledge about the effects of antidiabetic drugs on gut microbiota diversity and composition as well as the potential beneficial effects mediated by specific taxa. Metformin is the first-line treatment against T2D. In addition to its glucose-lowering and insulin sensitizing effects, metformin promotes SCFA-producing and mucin-degrading bacteria. Other antidiabetic drugs discussed in this review show positive effects on dysbiosis; but without any consensus specifically regarding the Firmicutes to Bacteroidetes ratio. Thus, beneficial effects might be mediated by specific taxa.