Prodrugs of phosphates, phosphonates, and phosphinates

Determination of the absolute configuration of phosphinic analogues of glutamate

Stereomers of LSP1-2111 have been separated, their absolute configuration and agonist activity at mGlu4R determined.

Prodrugs of phosphonates and phosphates: crossing the membrane barrier

A substantial portion of metabolism involves transformation of phosphate esters, including pathways leading to nucleotides and oligonucleotides, carbohydrates, isoprenoids and steroids, and phosphorylated proteins. Because the natural substrates bear one or more negative charges, drugs that target these enzymes generally must be charged as well, but small charged molecules can have difficulty traversing the cell membrane by means other than endocytosis. The resulting dichotomy has stimulated a great deal of effort to develop effective prodrugs, compounds that carry little or no charge to enable them to transit biological membranes, but able to release the parent drug once inside the target cell. This chapter presents recent studies on advances in prodrug forms, along with representative examples of their application to marketed and developmental drugs.

A highly water soluble benzimidazole derivative useful for the treatment of fasciolosis

This study describes the synthesis of compound (7), a highly hydrosoluble phosphonooxymethyl prodrug of compound alpha (4). Compound (7) improved the aqueous solubility of its precursor compound (4) by 50,000 times and it is stable at neutral pH. The prodrug showed faciolicidal activity when evaluated in vitro against excysted Fasciola hepatica metacercariae. The in vivo evaluation of (7) was carried out via oral, intramuscular and subcutaneous administration in sheep artificially infected with F. hepatica metacercariae. At an intramuscular dose of 4 mg/kg, the activity of (7) was similar to that of compound alpha (4) at an oral dose of 15 mg/kg.

Hydroxy fatty acids for the delivery of dideoxynucleosides as anti-HIV agents

A series of α- and β-carboxylated phospholipid prodrugs of dideoxy nucleosides have been synthesized and evaluated against HIV. An increase in biological effect with a factor of 500 has only been observed for the adenine nucleoside, which suggests that this prodrug approach is base specific.

Synthesis of phosphoramidate prodrugs: ProTide approach

The ProTide (pronucleotide) approach is a prodrug strategy elaborated to deliver nucleoside monophosphate into the cell, circumventing the first and inefficient rate-limiting phosphorylation step of nucleosides and improving the cellular penetration of nucleotides. The ProTide of a nucleoside phosphate is a phosphoramidate prodrug consisting of an amino acid ester promoiety linked via P-N bond to a nucleoside aryl phosphate. Such prodrugs have increased lipophilicity and thus are capable of altering cell and tissue distribution. The ProTide technology was successfully and extensively applied to a wide variety of nucleoside phosphates, endowed with antiviral and anticancer activity. This unit describes two different synthetic strategies allowing the preparation of phosphoramidates of 6-O-methyl-2'-β-C-methylguanosine as model compounds for nucleosides having only the 5'-OH as reactive hydroxyl group, and phosphoramidates of 2'-β-D-arabinouridine (AraU) as model compounds for nucleosides containing two or more reactive hydroxyl groups.

Development of oseltamivir phosphonate congeners as anti-influenza agents

Oseltamivir phosphonic acid (tamiphosphor, 3a), its monoethyl ester (3c), guanidino-tamiphosphor (4a), and its monoethyl ester (4c) are potent inhibitors of influenza neuraminidases. They inhibit the replication of influenza viruses, including the oseltamivir-resistant H275Y strain, at low nanomolar to picomolar levels, and significantly protect mice from infection with lethal doses of influenza viruses when orally administered with 1 mg/kg or higher doses. These compounds are stable in simulated gastric fluid, liver microsomes, and human blood and are largely free from binding to plasma proteins. Pharmacokinetic properties of these inhibitors are thoroughly studied in dogs, rats, and mice. The absolute oral bioavailability of these compounds was lower than 12%. No conversion of monoester 4c to phosphonic acid 4a was observed in rats after intravenous administration, but partial conversion of 4c was observed with oral administration. Advanced formulation may be investigated to develop these new anti-influenza agents for better therapeutic use.

Design, synthesis, and evaluation of novel cyclic phosphates of 5-aminosalicylic acid as cytochrome p450-activated prodrugs

Four novel cyclic phosphates of the anti-inflammatory agent 5-aminosalicylic acid (5-ASA) were designed and synthesized as cytochrome P450 (CYP)-activated prodrugs. These prodrugs can be used for targeting into gut wall, since these types of cyclic phosphates are known to be activated mainly by CYP3A forms, which are expressed not only in the liver but also in the small intestine and to a lesser extent in the colon. The present study shows that aromatic ring activating substituents, like chlorine, are definitely needed to obtain the desired enzymatic cleavage of the cyclic phosphate prodrugs of 5-ASA. However, the position of the activating substituent has also a strong impact on the chemical stability, and therefore, an appropriate balance between the rates of prodrug bioactivation and chemical stability needs to be taken into consideration in future studies on cyclic phosphate prodrugs of 5-ASA.

Structure-Activity Studies of Phosphopeptidomimetic Prodrugs Targeting the Src Homology 2 (SH2) Domain of Signal Transducer and Activator of Transcription 3 (Stat3)

Signal transducer and activator of transcription 3 (Stat3) transmits signals from growth factors and interleukin-6 family cytokines by binding to their receptors via its Src homology 2 (SH2) domain. This results in phosphorylation of Tyr705, dimerization, translocation to the nucleus, and regulation of transcription of downstream genes. Stat3 is constitutively activated in several human cancers and is a target for anti-cancer drug design. We have shown previously phosphorylation of Tyr705 in intact cancer cells can be inhibited with prodrugs of phosphopeptide mimics targeting the SH2 domain. In a series of prodrugs consisting of bis-pivaloyloxymethyl esters of 4'-phosphonodifluoromethyl cinnamoyl-Haic-Gln-NHBn, appending methyl group to the β-position of the cinnamate increased potency ca. twofold, which paralleled the increase in affinity of the corresponding phosphopeptide models. However, dramatic increases in potency were observed when the C-terminal C(O)NHBn of Gln-NHBn was replaced with a simple methyl group. In this communication we continue to explore the effects of structural modifications of prodrugs on their ability to inhibit Tyr705 phosphorylation. A set of 4-substituted prolines incorporated into β-methyl-4-phosphocinnamoyl-leucinyl-Xaa-4-aminopentamide model peptides exhibited affinities of 88-317 nM by fluorescence polarization (Pro IC₅₀ = 156 nM). In corresponding prodrugs, Pro inhibited constitutive Stat3 phosphorylation at 10 μM in MDA-MB-468 breast tumor cells. However, 4,4-difluoroproline and 4,4-dimethylproline resulted in complete inhibition at 0.5 μM. These results suggest that the prodrug with native proline undergoes metabolism that those with substituted prolines do not. In conclusion, changes in structure with minimal impact on intrinsic affinity can nevertheless have profound effects on the cellular potency of prodrug inhibitors of Stat3.

Medicinal Chemistry of Nucleoside Phosphonate Prodrugs for Antiviral Therapy

Considerable attention has been focused on the development of phosphonate-containing drugs for application in many therapeutic areas. However, phosphonate diacids are deprotonated at physiological pH and thus phosphonate-containing drugs are not ideal for oral administration, an extremely desirable requisite for the treatment of chronic diseases. To overcome this limitation several prodrug structures of biologically active phosphonate analogues have been developed. The rationale behind the design of such agents is to achieve temporary blockade of the free phosphonic functional group until their systemic absorption and delivery, allowing the release of the active drug only once at the target. In this paper, an overview of acyclic and cyclic nucleoside phosphonate prodrugs, designed as antiviral agents, is presented.

A practical synthesis of zanamivir phosphonate congeners with potent anti-influenza activity

Two phosphonate compounds 1a (4-amino-1-phosphono-DANA) and 1b (phosphono-zanamivir) are synthesized and shown more potent than zanamivir against the neuraminidases of avian and human influenza viruses, including the oseltamivir-resistant strains. For the first time, the practical synthesis of these phosphonate compounds is realized by conversion of sialic acid to peracetylated phosphono-DANA diethyl ester (5) as a key intermediate in three steps by a novel approach. In comparison with zanamivir, the high affinity of 1a and 1b can be partly attributable to the strong electrostatic interactions of their phosphonate groups with the three arginine residues (Arg118, Arg292, and Arg371) in the active site of neuraminidases. These phosphonates are nontoxic to the human 293T cells; they protect cells from influenza virus infection with EC(50) values in low-nanomolar range, including the wild-type WSN (H1N1), the 2009 pandemic (H1N1), the oseltamivir-resistant H274Y (H1N1), RG14 (H5N1), and Udorn (H3N2) influenza strains.

Fluorescence probe for lysophospholipase C/NPP6 activity and a potent NPP6 inhibitor

Nucleotide pyrophosphatases/phosphodiesterases (NPPs) are ubiquitous membrane-associated or secreted ectoenzymes that have a role in regulating extracellular nucleotide and phospholipid metabolism. Among the members of the NPP family, NPP1 and -3 act on nucleotides such as ATP, while NPP2, -6, and -7 act on phospholipids such as lysophosphatidylcholine and sphingomyelin. NPP6, a recently characterized NPP family member, is a choline-specific glycerophosphodiester phosphodiesterase, but its functions remain to be analyzed, partly due to the lack of highly sensitive activity assay systems and practical inhibitors. Here we report synthesis of novel NPP6 fluorescence probes, TG-mPC and its analogues TG-mPC(3)C, TG-mPC(5)C, TG-mPENE, TG-mPEA, TG-mPhos, TG-mPA, TG-mPMe, and TG-mPPr. Among the seven NPPs, only NPP6 hydrolyzed TG-mPC, TG-mPC(3)C, and TG-mPENE. TG-mPC was hydrolyzed in the cell lysate from NPP6-transfected cells, but not control cells, showing that it is suitable for use in cell-based NPP6 assays. We also examined the usefulness of TG-mPC as a fluorescence imaging probe. We further applied TG-mPC to carry out high-throughput NPP6 inhibitor screening and found several NPP6-selective inhibitors in a library of about 80,000 compounds. Through structure-activity relationship (SAR) analysis, we identified a potent and selective NPP6 inhibitor with an IC(50) value of 0.21 μM. Our NPP6-selective fluorescence probe, TG-mPC, and the inhibitor are expected to be useful to elucidate the biological function of NPP6.

Prodrugs--from serendipity to rational design

The prodrug concept has been used to improve undesirable properties of drugs since the late 19th century, although it was only at the end of the 1950s that the actual term prodrug was introduced for the first time. Prodrugs are inactive, bioreversible derivatives of active drug molecules that must undergo an enzymatic and/or chemical transformation in vivo to release the active parent drug, which can then elicit its desired pharmacological effect in the body. In most cases, prodrugs are simple chemical derivatives that are only one or two chemical or enzymatic steps away from the active parent drug. However, some prodrugs lack an obvious carrier or promoiety but instead result from a molecular modification of the prodrug itself, which generates a new active compound. Numerous prodrugs designed to overcome formulation, delivery, and toxicity barriers to drug utilization have reached the market. In fact, approximately 20% of all small molecular drugs approved during the period 2000 to 2008 were prodrugs. Although the development of a prodrug can be very challenging, the prodrug approach represents a feasible way to improve the erratic properties of investigational drugs or drugs already on the market. This review introduces in depth the rationale behind the use of the prodrug approach from past to present, and also considers the possible problems that can arise from inadequate activation of prodrugs.

Identification of amino acid phosphorodiamidates of antiviral nucleosides using electrospray ionization tandem mass spectrometry

Several amino acid phosphorodiamidate derivatives of d4T as anti-HIV prodrugs were synthesized and investigated using electrospray ionization multistage tandem mass spectrometry (ESI-MS(n)). A novel methyl group migration in gas phase was observed in ESI-MS(2) of the sodium adducts of amino acid methyl ester of phosphorodiamidates of 2',3'-didehydro-2',3'-dideoxythymidine (d4T). The proposed structures of the rearrangement ions were confirmed by high resolution tandem mass spectrometry. A possible mechanism involving the pentacoordinate phosphoric-carboxylic phosphate anhydride was proposed, in which a seven-membered ring intermediate was formed by coordination with the metal ion between the phosphoryl group and carbonyl oxygen atom. Thus, the intrinsic properties of phosphoryl group might be the key factors responsible for this migration.

On the existence of the chair conformation in six-membered ring phosphates bearing an aryl group axially oriented at the C4 position: cyclic nucleotides as model compounds for cyclic phosph(on)ate and phosphoramide prodrugs

A series of cyclic nucleotide analogues to HepDirect prodrugs were prepared by a three-component reaction of protected thymine, phosphoryl chloride, and 5-aryl-alpha-D-xylofuranoses derivatives. One of the cyclic nucleotides showed NMR data that suggest a predominant twisted conformation; however, in spite of having an aryl group at the C4 position within the crystal lattice, the cyclic nucleotide had a chair conformation with the aryl group axially oriented. By analyzing the unprecedented X-ray structure, it was observed that the oxygen atom from the phoshoryl group (P=O) is found in close proximity to the o-hydrogen atom of the aryl group (2.51 A), suggesting thus an attractive nonbonding electrostatic interaction, which might be the driving force that overcomes the steric diaxial interactions imposed by the aryl group. Theoretical studies (NBO) for two model compounds showed that there are indeed interactions between filled (donor) Lewis-type NBOs and empty (acceptor) non-Lewis NBOs corresponding to the nO-->sigma*(C-H) interaction. Additionally, conversion of a diastereomeric mixture of cyclic nucleotides into the more stable diastereomeric cyclic nucleotide was observed and explained by spontaneous isomerization in the phosphorinane ring. This finding supports the recently established hypothesis for the mode of action of prodrug cleavage, for which the anomeric effect plays an important role.

ATP chemosensitivity testing of new antitumor duplex drugs linking 3`-C-ethynylycytidine (ECyd) and 2´-deoxy-5-fluorouridine (5-FdU) in comparison to standard cytostatica and combinations thereof

BACKGROUND

2´-Deoxy-5-fluorouridylyl-(5´-5´)-3´-C-ethynylcytidine [5-FdU(5´-5´)ECyd] and 3´-C-ethynylcytidinylyl-(5´->1-O)-2-O-octadecyl-sn-glycerylyl-(3-Ο->5´)-2´-deoxy-5-fluorouridine [ECyd-lipid-5-FdU] are antitumor active duplex drugs and these heterodinucleoside phosphate analogues could be cleaved in vivo by wide-spread phosphodiesterases into different antitumor active antimetabolites.

METHODS

We cultured breast MCF-7, MDA-MB-231 and ovarian OVCAR-29 and OAW-42 cancer cell lines and used the luminometric measuring of the ATP tumor chemosensitivity assay to assess the in vitro activity of 5-FdU(5´-5´)ECyd and ECyd-lipid-5-FdU in comparison to standard single cytostatic agents and combinations thereof currently used in anticancer therapies. To allow comparison between samples and different regimens IndexSUM was determined based on the percentage tumor cell growth inhibition at each test drug concentration. Additionally, the cytostatic efficacy of 5-FdU(5´-5´)ECyd and ECyd-lipid-5-FdU was evaluated at a minimum of five concentrations at 10 fold dilutions using 60 human tumor cell lines including ovarian and breast cancer cell lines from the National Cancer Institute (USA).

RESULTS

5-FdU(5´-5´)ECyd and ECyd-lipid-5-FdU have a high cytostatic efficacy reaching 50% tumor cell growth inhibition at concentrations ranging between nano- and micomolar. IndexSum values for broad range efficacy in MCF-7 breast cancer cells were comparable to values obtained for standard drug combinations. Higher cytostatic efficacy was observed in MDA-MB-231 cells.

CONCLUSION

The duplex drugs 5-FdU(5´-5´)ECyd and ECyd-lipid-5-FdU represent potential new chemotherapeutic drugs for breast and ovarian cancer cells which are comparable to currently used drug combinations and more potent in comparison to some monocytostatica used in cancer therapy.

Synthesis and in vitro evaluation of aspartate transcarbamoylase inhibitors

The design, synthesis, and evaluation of a series of novel inhibitors of aspartate transcarbamoylase (ATCase) are reported. Several submicromolar phosphorus-containing inhibitors are described, but all-carboxylate compounds are inactive. Compounds were synthesized to probe the postulated cyclic transition-state of the enzyme-catalyzed reaction. In addition, the associated role of the protonation state at the phosphorus acid moiety was evaluated using phosphinic and carboxylic acids. Although none of the synthesized inhibitors is more potent than N-phosphonacetyl-l-aspartate (PALA), the compounds provide useful mechanistic information, as well as the basis for the design of future inhibitors and/or prodrugs.

Design, synthesis and evaluation of novel oxazaphosphorine prodrugs of 9-(2-phosphonomethoxyethyl)adenine (PMEA, adefovir) as potent HBV inhibitors

A series of novel oxazaphosphorine prodrugs of 9-(2-phosphonomethoxyethyl)adenine (PMEA, adefovir) were synthesized and their anti-hepatitis B virus (HBV) activity was evaluated in HepG2 2.2.15 cells, with adefovir dipivoxil as a reference drug. In the cell assays, compounds 7b and 7d exhibited anti-HBV activity comparable to that of adefovir dipivoxil, while compound 7c, with an IC(50) value of 0.12 microM, was found to be three times more potent than the reference compound. In vitro stability studies showed that (S(P),S)-7c, the diastereomer of compound 7c, was stable in human blood plasma but underwent rapid metabolism to release the parent drug PMEA in liver microsomes. The possible metabolic pathway of (S(P),S)-7c in human liver microsomes was described. These findings suggest that compound (S(P),S)-7c is a promising anti-HBV drug candidate for further development.

Selective nucleoside triphosphate diphosphohydrolase-2 (NTPDase2) inhibitors: nucleotide mimetics derived from uridine-5'-carboxamide

Ecto-nucleoside triphosphate diphosphohydrolases (E-NTPDases, subtypes 1, 2, 3, 8 of NTPDases) dephosphorylate nucleoside tri- and diphosphates to the corresponding di- and monophosphates. In the present study we synthesized adenine and uracil nucleotide mimetics, in which the phosphate residues were replaced by phosphonic acid esters attached to the nucleoside at the 5'-position by amide linkers. Among the synthesized uridine derivatives, we identified the first potent and selective inhibitors of human NTPDase2. The most potent compound was 19a (PSB-6426), which was a competitive inhibitor of NTPDase2 exhibiting a K i value of 8.2 microM and selectivity versus other NTPDases. It was inactive toward uracil nucleotide-activated P2Y 2, P2Y 4, and P2Y 6 receptor subtypes. Compound 19a was chemically and metabolically highly stable. In contrast to the few known (unselective) NTPDase inhibitors, 19a is an uncharged molecule and may be perorally bioavailable. NTPDase2 inhibitors have potential as novel cardioprotective drugs for the treatment of stroke and for cancer therapy.

Catalytic asymmetric stetter reaction onto vinylphosphine oxides and vinylphosphonates

An intramolecular Stetter reaction of vinylphosphine oxides and vinylphosphonates has been developed. Treatment of an aldehyde with a nucleophilic N-heterocyclic carbene catalyst allows for addition of an acyl anion equivalent into a vinylphosphine oxide or vinylphosphonate Michael acceptor in yields up to 99% and ee values up to 96%.

Prodrugs of nucleoside analogues for improved oral absorption and tissue targeting

Nucleoside analogues are widely used for the treatment of antiviral infections and anticancer chemotherapy. However, many nucleoside analogues suffer from poor oral bioavailability due to their high polarity and low intestinal permeability. In order to improve oral absorption of these polar drugs, prodrugs have been employed to increase lipophilicity by chemical modification of the parent. Alternatively, prodrugs targeting transporters present in the intestine have been exploited to facilitate the transport of the nucleoside analogues. Valacyclovir and valganciclovir are two successful valine ester prodrugs transported by the PepT1 transporter. Recently, research efforts have focused on design of prodrugs for tissue specific delivery to improve efficacy and safety. This review presents advances of prodrug approaches for improved oral absorption of nucleoside analogues and recent developments in tissue targeting.

Pradefovir: a prodrug that targets adefovir to the liver for the treatment of hepatitis B

Adefovir dipivoxil, a marketed drug for the treatment of hepatitis B, is dosed at submaximally efficacious doses because of renal toxicity. In an effort to improve the therapeutic index of adefovir, 1-aryl-1,3-propanyl prodrugs were synthesized with the rationale that this selectively liver-activated prodrug class would enhance liver levels of the active metabolite adefovir diphosphate (ADV-DP) and/or decrease kidney exposure. The lead prodrug (14, MB06866, pradefovir), identified from a variety of in vitro and in vivo assays, exhibited good oral bioavailability (F = 42%, mesylate salt, rat) and rate of prodrug conversion to ADV-DP. Tissue distribution studies in the rat using radiolabeled materials showed that cyclic 1-aryl-1,3-propanyl prodrugs enhance the delivery of adefovir and its metabolites to the liver, with pradefovir exhibiting a 12-fold improvement in the liver/kidney ratio over adefovir dipivoxil.

Phenylphosphonate transport by Helicobacter pylori

BACKGROUND

Helicobacter pylori can utilize phenylphosphonate as a sole source of phosphorus, and it is able to transport the phosphonate N-phosphonoacetyl-L-aspartate. However, H. pylori does not have any genes homologous to those of the known pathways for phosphonate degradation in bacteria, indicating that it must have novel pathways for the transport and metabolism of phosphonates.

METHODS

Phenylphosphonate transport by H. pylori was studied in strains LC20, J99 and N6 by the centrifugation through oil method using [(14)C]-labeled phenylphosphonate.

RESULTS

The Michaelis constants of transport K(t) and V(max) for phenylphosphonate showed similar kinetics in the three strains. The Arrhenius plot for phenylphosphonate transport rates at permeant concentrations of 50 micromol/L was linear over the temperature range 10-40 degrees C with an activation energy of 3.5 kJ/mol, and a breakpoint between 5 and 10 degrees C. Transport rates increased with monovalent cation size. The effects of various inhibitors were investigated: iodoacetamide, amiloride, valinomycin, and nigericin reduced the rate of phenylphosphonate transport; sodium azide and sodium cyanide increased the transport rate; and monensin had no effect.

CONCLUSIONS

The kinetics and properties of H. pylori phenylphosphonate transport were characterized, and the data suggested a carrier-mediated transport mechanism.

A novel approach to phosphonic acids from hypophosphorous acid

A novel access to phosphonic acids via Pd-catalyzed tandem carbon-phosphorus bond formation - oxidation processes was developed. The procedures involve atom-economical and environmentally friendly functionalization reactions of hypophosphorous acid (H(3)PO(2)) and H-phosphinic acids [RP(O)(OH)(H)].