Phosphinic acids: current status and potential for drug discovery

Phosphinic acid derivatives exhibit diverse biological activities and a high degree of structural diversity, rendering them a versatile tool in the development of new medicinal agents. Pronounced recent progress, coupled with previous research findings, highlights the impact of this moiety in medicinal chemistry. Here, we highlight the most important breakthroughs made with phosphinates with a range of pharmacological activities against many diseases, including anti-inflammatory, anti-Alzheimer, antiparasitic, antihepatitis, antiproliferative, anti-influenza, anti-HIV, antimalarial, and antimicrobial agents. We also provide the current status of the corresponding prodrugs, drug-delivery systems, and drug applications of phosphinic acids in the clinical stage.

Aminophosphinates against Helicobacter pylori ureolysis-Biochemical and whole-cell inhibition characteristics

Urease is an important virulence factor from Helicobacter pylori that enables bacterial colonization of human gastric mucosa. Specific inhibition of urease activity can be regarded as a promising adjuvant strategy for eradication of this pathogen. A group of organophosphorus inhibitors of urease, namely, aminophosphinic acid and aminophosphonic acid derivatives, were evaluated in vitro against H. pylori urease. The kinetic characteristics of recombinant enzyme activity demonstrated a competitive reversible mode of inhibition with K_i values ranging from 0.294 to 878 μM. N-n-Hexylaminomethyl-P-aminomethylphosphinic acid and N-methylaminomethyl-P-hydroxymethylphosphinic acid were the most effective inhibitors (K_i = 0.294 μM and 1.032 μM, respectively, compared to K_i = 23 μM for the established urease inhibitor acetohydroxamic acid).

The biological relevance of the inhibitors was verified in vitro against a ureolytically active Escherichia coli Rosetta host that expressed H. pylori urease and against a reference strain, H. pylori J99 (CagA⁺/VacA⁺). The majority of the studied compounds exhibited urease-inhibiting activity in these whole-cell systems. Bis(N-methylaminomethyl)phosphinic acid was found to be the most effective inhibitor in the susceptibility profile studies of H. pylori J99. The cytotoxicity of nine structurally varied inhibitors was evaluated against four normal human cell lines and was found to be negligible.

Repurposing Lesogaberan to Promote Human Islet Cell Survival and β-Cell Replication

The activation of β-cell's A- and B-type gamma-aminobutyric acid receptors (GABA_A-Rs and GABA_B-Rs) can promote their survival and replication, and the activation of α-cell GABA_A-Rs promotes their conversion into β-cells. However, GABA and the most clinically applicable GABA-R ligands may be suboptimal for the long-term treatment of diabetes due to their pharmacological properties or potential side-effects on the central nervous system (CNS). Lesogaberan (AZD3355) is a peripherally restricted high-affinity GABA_B-R-specific agonist, originally developed for the treatment of gastroesophageal reflux disease (GERD) that appears to be safe for human use. This study tested the hypothesis that lesogaberan could be repurposed to promote human islet cell survival and β-cell replication. Treatment with lesogaberan significantly enhanced replication of human islet cells in vitro, which was abrogated by a GABA_B-R antagonist. Immunohistochemical analysis of human islets that were grafted into immune-deficient mice revealed that oral treatment with lesogaberan promoted human β-cell replication and islet cell survival in vivo as effectively as GABA (which activates both GABA_A-Rs and GABA_B-Rs), perhaps because of its more favorable pharmacokinetics. Lesogaberan may be a promising drug candidate for clinical studies of diabetes intervention and islet transplantation.

[Therapy of gastroesophageal reflux disease (GERD)]

Gastroesophageal reflux disease (GERD) comprises all symptoms and clinical consequences in the context of reflux of stomach contents into the esophagus. The symptoms reported by patients include heartburn, regurgitation and sour taste in the mouth. In some cases atypical reflux associated symptoms such as asthma, laryngitis or recurrent pneumonias are reported. Pathophysiologically an incompetence of the lower esophageal sphincter and a disturbed clearance of the esophagus are the underlying mechanisms. Current treatment recommendations include a change of lifestyle and a drug treatment with proton pump inhibitors (PPI) being widely used. In patients with persistent symptoms other diagnoses like functional dyspepsia should be considered especially when additional symptoms like epigastric pain, postprandial fullness and nausea are present. This review summarizes the current understanding of the pathophysiology, the diagnosis and the treatment of GERD and gives an outlook on therapies currently developed for the treatment of reflux disease. A promising new drug, presently classified as being a reflux inhibitor, is lesogaberan. Lesogaberan is presently studied in phase II clinical trials.

IDX-899, an aryl phosphinate-indole non-nucleoside reverse transcriptase inhibitor for the potential treatment of HIV infection

Antiretroviral drug resistance is one of the complications of highly active antiretroviral therapy (HAART). Second-generation non-nucleoside reverse transcriptase inhibitors (NNRTIs) are being developed for use in patients infected with HIV-1 because of the enhanced activity of these inhibitors against viruses that are resistant to the first-generation NNRTIs. IDX-899 (GSK-2248761), under development by Idenix Pharmaceuticals Inc and ViiV Healthcare, is an aryl phosphinate-indole second-generation NNRTI that potently and selectively inhibits wild-type and NNRTI-resistant HIV-1 in vitro. Preclinical data for IDX-899 suggest a significantly greater barrier to resistance compared with that of the first-generation NNRTI efavirenz. Two pathways of resistance have been identified for IDX-899 in vitro that include Glu138Lys and Val90Ile/Tyr181Cys mutations. Pharmacokinetic studies demonstrated that IDX-899 exhibits linear, dose-proportional and food-dependent pharmacokinetics; Cmin concentrations achieved with this drug allow once-daily dosing. In phase I clinical trials, IDX-899 reduced HIV-1 RNA and increased CD4+ cell counts in treatment-naïve patients infected with HIV-1. At the time of publication, a phase II clinical trial of IDX-899 was being conducted.

Blockade of GABA(B) receptors completely reverses age-related learning impairment

Impaired cognitive functions are well-described in the aging process. GABA(B) antagonists can facilitate learning and memory in young subjects, but these agents have not been well-characterized in aging. Here we show a complete reversal of olfactory discrimination learning deficits in cognitively-impaired aged Fischer 344 rats using the GABA(B) antagonist CGP55845, such that drug treatment restored performance to that on par with young and cognitively-unimpaired aged subjects. There was no evidence that this improved learning was due to enhanced olfactory detection abilities produced by the drug. These results highlight the potential of targeting GABA(B) receptors to ameliorate age-related cognitive deficits and demonstrate the utility of olfactory discrimination learning as a preclinical model for testing novel therapies to improve cognitive functions in aging.

Improving non-nucleoside reverse transcriptase inhibitors for first-line treatment of HIV infection: the development pipeline and recent clinical data

Efavirenz non-nucleoside reverse transcriptase inhibitor (NNRTI)-based therapy or boosted protease inhibitor (PI)-based therapy are currently recommended as first-line regimens for the treatment of HIV infection. Although the available therapy options are efficacious and well-tolerated in the majority of patients, treatment durability is still limited by drug-related side effects, inadequate patient adherence and the development of drug resistance. PI-based regimens have higher tablet loads, more complicated drug interactions and have been associated with gastrointestinal side effects and metabolic abnormalities. NNRTI-based regimens are efficacious, but have a low genetic barrier to resistance and have been associated with rash, hypersensitivity reactions and central nervous system side effects. There is, therefore, still a need for first-line antiviral agents that facilitate patient adherence and allow durable suppression of viral replication. The next-generation NNRTIs in development include rilpivirine (TMC-278), UK-453061, RDEA-806, IDX-899 and MK-4965. These NNRTIs demonstrate significant advantages over efavirenz, and may improve treatment options for first-line therapy. A number of other structurally diverse compounds that inhibit common NNRTI-resistant mutant viruses are also under investigation. In this review, the desirable features of a next-generation NNRTI for treatment-naïve patients are discussed, as well as the properties of the NNRTIs that are currently in development.

TAFIa inhibitors in the treatment of thrombosis

Since activated thrombin-activatable fibrinolysis inhibitor (TAFIa) was discovered in 1988, considerable interest has developed in the biological role of this enzyme, particularly in hemostasis and thrombotic diseases. Given the large number of publications about the underpinning biology of fibrinolysis, the relatively small number of reported chemical tools or drug-like molecules that target this mechanism is surprising. In the context of drug design, most of the disclosed fibrinolysis inhibitors occupy less-exploited regions of drug-like space. To generalize, most of these molecules are either small and hydrophilic, or are larger carboxylic acids. The chemical nature of these inhibitors reflects those of the endogenous substrate for this zinc metalloprotease enzyme target. Knowledge of the target has defined the way medicinal chemists have designed inhibitors to target this enzyme of intriguing therapeutic potential. This review summarizes the publications, patent literature and company disclosures on small-molecule inhibitors of TAFIa from 2006 to the present. Selected significant disclosures prior to this period are also highlighted.

Progress in the discovery and development of glutamate carboxypeptidase II inhibitors

During the past 10 years, substantial progress has been made in the discovery and development of small molecule glutamate carboxypeptidase II (GCP II) inhibitors. These inhibitors have provided the necessary tools to investigate the physiological role of GCP II as well as the potential therapeutic benefits of its inhibition in neurological disorders of glutamatergic dysregulation. This review article details key GCP II inhibitors discovered in the last decade and important findings from preclinical and clinical studies.

The M18 aspartyl aminopeptidase of the human malaria parasite Plasmodium falciparum

A member of the M18 family of aspartyl aminopeptidases is expressed by all intra-erythrocytic stages of the human malaria parasite Plasmodium falciparum (PfM18AAP), with highest expression levels in rings. Functionally active recombinant enzyme, rPfM18AAP, and native enzyme in cytosolic extracts of malaria parasites are 560-kDa octomers that exhibit optimal activity at neutral pH and require the presence of metal ions to maintain enzymatic activity and stability. Like the human aspartyl aminopeptidase, the exopeptidase activity of PfM18AAP is exclusive to N-terminal acidic amino acids, glutamate and aspartate, making this enzyme of particular interest and suggesting that it may function alongside the malaria cytosolic neutral aminopeptidases in the release of amino acids from host hemoglobin-derived peptides. Whereas immunocytochemical studies using transgenic P. falciparum parasites show that PfM18AAP is expressed in the cytosol, immunoblotting experiments revealed that the enzyme is also trafficked out of the parasite into the surrounding parasitophorous vacuole. Antisense-mediated knockdown of PfM18AAP results in a lethal phenotype as a result of significant intracellular damage and validates this enzyme as a target at which novel antimalarial drugs could be directed. Novel phosphinic derivatives of aspartate and glutamate showed modest inhibition of rPfM18AAP but did not inhibit malaria growth in culture. However, we were able to draw valuable observations concerning the structure-activity relationship of these inhibitors that can be employed in future inhibitor optimization studies.

CGP 56999A, a GABA(B) receptor antagonist, enhances expression of brain-derived neurotrophic factor and attenuates dopamine depletion in the rat corpus striatum following a 6-hydroxydopamine lesion of the nigrostriatal pathway

Rats were injected (i.p.) once daily with either 1 mg/kg CGP 56999A, a gamma-aminobutyric acid(B) (GABA(B)) receptor antagonist, or an equivalent volume of saline beginning 7 days prior to, and continuing for 7 days following, a unilateral 6-hydroxydopamine lesion of the nigrostriatal dopamine (DA) pathway. At the end of the CGP 56999A treatment period the concentrations of DA and dihydroxyphenylacetic acid (DOPAC), as well as the expression of brain-derived neurotrophic factor (BDNF), were analyzed in corpus striatum ipsilateral and contralateral to the lesioning. No significant differences in these parameters were noted in the contralateral striatum between saline- and CGP 56999A-treated subjects. In contrast, as compared to animals receiving saline only, daily treatment with the GABA(B) receptor antagonist significantly attenuated the 6-hydroxydopamine-induced decline in DA and increased the expression of BDNF in the ipsilateral striatum. The results indicate that CGP 56999A enhances BDNF gene expression in the rat corpus striatum and prevents the decline in DA content that is a characteristic sequela of 6-hydroxydopapmine lesions of the nigrostraital dopamine pathway. These findings suggest that GABA(B) receptor antagonists may be of value in the treatment of Parkinson's disease and other conditions that would benefit from an enhanced production of neurotrophic factors in brain.

Inhibition of angiotensin I–converting enzyme induces radioprotection by preserving murine hematopoietic short-term reconstituting cells

Angiotensin I–converting enzyme (ACE) inhibitors can affect hematopoiesis by several mechanisms including inhibition of angiotensin II formation and increasing plasma concentrations of AcSDKP (acetyl-N-Ser-Asp-Lys-Pro), an ACE substrate and a negative regulator of hematopoiesis. We tested whether ACE inhibition could decrease the hematopoietic toxicity of lethal or sublethal irradiation protocols. In all cases, short treatment with the ACE inhibitor perindopril protected against irradiation-induced death. ACE inhibition accelerated hematopoietic recovery and led to a significant increase in platelet and red cell counts. Pretreatment with perindopril increased bone marrow cellularity and the number of hematopoietic progenitors (granulocyte macrophage colony-forming unit [CFU-GM], erythroid burst-forming unit [BFU-E], and megakaryocyte colony-forming unit [CFU-MK]) from day 7 to 28 after irradiation. Perindopril also increased the number of hematopoietic stem cells with at least a short-term reconstitutive activity in animals that recovered from irradiation. To determine the mechanism of action involved, we evaluated the effects of increasing AcSDKP plasma concentrations and of an angiotensin II type 1 (AT1) receptor antagonist (telmisartan) on radioprotection. We found that the AT1-receptor antagonism mediated similar radioprotection as the ACE inhibitor. These results suggest that ACE inhibitors and AT1-receptor antagonists could be used to decrease the hematopoietic toxicity of irradiation.

Enhancement of fibrinolysis by EF6265 [(S)-7-amino-2-[[[(R)-2-methyl-1-(3-phenylpropanoylamino)propyl]hydroxyphosphinoyl] methyl]heptanoic acid], a specific inhibitor of plasma carboxypeptidase B

Plasma procarboxypeptidase B, also known as thrombin-activatable fibrinolysis inhibitor (TAFI), is converted by thrombin into the active enzyme, carboxypeptidase B (CPB)/activated TAFI. Plasma CPB down-regulates fibrinolysis by removing carboxy-terminal lysines, the ligands for plasminogen and tissue-type plasminogen activator (tPA), from partially degraded fibrin. To target thrombosis in a new way, we have identified and optimized a phosphinic acid-containing inhibitor of CPB, EF6265 [(S)-7-amino-2-[[[(R)-2-methyl-1-(3-phenylpropanoylamino) propyl]hydroxyphosphinoyl]methyl]heptanoic acid] and determined both the pharmacological profile and pathophysiological role of CPB in rat thrombolysis. EF6265 specifically inhibited plasma CPB activity with an IC(50) (50% inhibitory concentration) of 8.3 nM and enhanced tPA-mediated clot lysis in a concentration-dependent manner. EF6265 decreased detectable thrombi (percentage of glomerular fibrin deposition; control, 98 +/- 1.1; EF6265, 0.1 mg/kg, 27 +/- 9.1) that had been generated by tissue factor in a rat microthrombosis model with concomitant increases in plasma D-dimer concentration (control, <0.5 microg/ml; EF6265, 0.1 mg/kg, 15 +/- 3.5 microg/ml). EF6265 reduced plasma alpha2-antiplasmin activity to a lesser extent than tPA. In an arteriovenous shunt model, EF6265 (1 mg/kg) enhanced exogenous tPA-mediated thrombolysis under the same conditions that neither EF6265 nor tPA (600 kIU/kg) alone reduced thrombi. EF6265 (1 and 30 mg/kg) did not affect the bleeding time in rats. Moreover, it did not prolong the bleeding time evoked by tPA (600 kIU/kg). These results confirm that circulating procarboxypeptidase B functions as a fibrinolysis inhibitor's zymogen and validates the use of CPB inhibitors as both an enhancer of physiological fibrinolysis in microcirculation and as a novel adjunctive agent to tPA for thromboembolic diseases while maintaining a small effect on primary hemostasis.

Comparative study of the anti-human cytomegalovirus activities and toxicities of a tetrahydrofuran phosphonate analogue of guanosine and cidofovir

Cidofovir is the first nucleoside monophosphate analogue currently being used for the treatment of human cytomegalovirus (HCMV) retinitis in individuals with AIDS. Unfortunately, the period of therapy with the use of this compound may be limited due to the possible emergence of serious irreversible nephrotoxic effects. New drugs with improved toxicity profiles are needed. The goal of this study was to investigate the anticytomegaloviral properties and drug-induced toxicity of a novel phosphonate analogue, namely, (-)-2-(R)-dihydroxyphosphinoyl-5-(S)-(guanin-9'-yl-methyl) tetrahydrofuran (compound 1), in comparison with those of cidofovir. The inhibitory activities of both compounds on HCMV propagation in vitro were similar against the AD 169 and Towne strains, with 50% inhibitory concentrations ranging from 0.02 to 0.17 microgram/ml for cidofovir and < 0.05 to 0.09 microgram/ml for compound 1. A clinical HCMV isolate that was resistant to ganciclovir and that had a known mutation within the UL54 DNA polymerase gene and a cidofovir-resistant laboratory strain derived from strain AD 169 remained sensitive to compound 1, whereas their susceptibilities to ganciclovir and cidofovir were reduced by 33- and 10-fold, respectively. Both compound 1 and cidofovir exhibited equal potencies in an experimentally induced murine cytomegalovirus (MCMV) infection in mice, with a prevention or prolongation of mean day to death at dosages of 1.0, 3.2, and 10.0 mg/kg of body weight/day. In cytotoxicity experiments, compound 1 was found to be generally more toxic than cidofovir in cell lines Hs68, HFF, and 3T3-L1 (which are permissive for HCMV or MCMV replication) but less toxic than cidofovir in MRC-5 cells (which are permissive for HCMV replication). Drug-induced toxic side effects were noticed for both compounds in rats and guinea pigs in a 5-day repeated-dose study. In guinea pigs, a greater weight loss was noticed with cidofovir than with compound 1 at dosages of 3.0 and 10.0 mg/kg/day. An opposite effect was detected in rats, which were treated with the compounds at relatively high dosages (up to 100 mg/kg/day). Compound 1 and cidofovir were nephrotoxic in both rats and guinea pigs, with the epithelium lining the proximal convoluted tubules in the renal cortex being the primary target site. The incidence and the severity of the lesions were found to be dose dependent. The lesions observed were characterized by cytoplasm degeneration and nuclear modifications such as karyomegaly, the presence of pseudoinclusions, apoptosis, and degenerative changes. In the guinea pig model, a greater incidence and severity of lesions were observed for cidofovir than for compound 1 (P < 0.001) with a drug regimen of 10 mg/kg/day.

Opposite effects of GABAB receptor antagonists on absences and convulsive seizures

In Wistar rats with spontaneous non-convulsive absence epilepsy, absence seizures were dose dependently suppressed by intraperitoneal administration of the GABAB receptor antagonists CGP 36742, 50-400 mg/kg, and CGP 56999, 0.25-0.75 mg/kg, and by bilateral microinjections of the same compounds into the lateral nuclei of the thalamus. In rats susceptible to audiogenic seizures, intraperitoneal administration of both GABAB receptor antagonists, at doses which suppressed absence seizures, facilitated the elicitation of sound-induced tonic seizures. In non-epileptic control rats, intraperitoneal injections of higher doses of CGP 36742 (800-2400 mg/kg) and CGP 56999 (3-6 mg/kg) induced delayed clonic convulsions, which were suppressed by pretreatment with baclofen. c-Fos protein was expressed after GABAB receptor antagonist-induced seizures in the cortex, hippocampus, amygdala, perirhinal and piriform cortex. Intra-cortical and hippocampal microinfusion of both GABAB receptor antagonists produced focal seizures. In conclusion, GABAB receptor antagonists suppress non-convulsive absence seizures by blocking thalamic GABAB receptors, while they induce convulsions in cortical and limbic structures.

[Pain-alleviating action of gidifen and its combinations with analgesics]

The analgesic effect of gidifen, a new tranquilizer belonging to the group of organophosphorus compounds, manifests itself in relatively high doses (1/3 of the LD50). The quantitative characteristics of the analgesic action depends on the method for evaluating the analgesic action of the drug. On combined use of gidifen and analgesics applied in a definite dosage range the analgesic effect is potentiated. If the tranquilizer under study is combined with non-narcotic analgesics, the above potentiation is accompanied with an increase in the toxicity and myorelaxant activity of gidifen. On combination of the latter with morphine the parameters under consideration are unchanged. According to the pattern of interaction with analgesics gidifen does not differ in principle from benzodiazepine tranquilizers. However, the drug is not superior to benzodiazepines in this respect.

[Effect of gidifen on behavior and hemodynamic signs of an emotionally stressful reaction]

Gidifen (diphenylphosphinylacetic acid hydrazide) is an original Soviet tranquilizer. In doses of 120--150 mg/kg it restores central baroreflex regulation of the arterial pressure supressed during acute emotional-stress reaction in cats (confrontation with a dog). The drug has an antihypertensive action and reduces tachycardia caused by the emotional stress. It has a selective antiphobic and antiaggressive action and does not produce ataxia. Quantitative evaluation of behavior was performed during group interaction of animals according to the five-point system.

Novel phosphate anthelmintics. 3. Alkyl and aryl 1-methyleneallyl phosphates, phosphonates, and phosphinates

A series of new highly chlorinated 1-methyleneallyl ("butadienyl") dialkyl phosphates and related phosphonates and phosphinates has been synthesized and assessed for anthelmintic activity in mice against the tapeworm Hymenolepis nana and the pinworm Syphacia obvelata. Highest activity was observed with diethyl 2,3,3-trichloro-1-dichloromethyleneallyl ("perchlorobutadienyl") phosphate (14), while replacement of both ethoxy groups by methoxy and larger alkoxy or phenyl groups gave less efficacious compounds. In general, chlorine depletion of the 1,3-butadien-2-yl moiety or saturation of one double bond reduced anthelmintic responses.