Bisphosphonate prodrugs: synthesis and in vitro evaluation of novel acyloxyalkyl esters of clodronic acid

Molecular dynamics simulation study of DNA conformation changes caused by the dinuclear platinum(II) complexes with the bisphosphonate group

Bisphosphonate (BP) has been widely used as a bone-targeting group, and the BP-modified platinum(II) complexes have shown potential to as anticancer drugs against bone-related diseases, such as osteosarcoma. DNA conformation changes induced by the BP-modified dinuclear platinum(II) complexes have been investigated using molecular dynamics simulations. The results indicated that the BP-modified dinuclear platinum(II) complexes coordinated to DNA results in DNA structural distortions, including twisting, unwinding and bending. Furthermore, the rigidity of the bridging linkers in the BP-modified platinum(II) complex may induce more significant DNA structural distortions with same spans. The results provide the detail information of DNA conformational changes induced by the BP-modified platinum(II) complexes with different flexibility of bridging linkers, and are helpful for exploring novel platinum-based antitumor drugs.

Prodrugs of pyrophosphates and bisphosphonates: disguising phosphorus oxyanions

Pyrophosphates have important functions in living systems and thus pyrophosphate-containing molecules and their more stable bisphosphonate analogues have the potential to be used as drugs for treating many diseases including cancer and viral infections. Both pyrophosphates and bisphosphonates are polyanionic at physiological pH and, whilst this is essential for their biological activity, it also limits their use as therapeutic agents. In particular, the high negative charge density of these compounds prohibits cell entry other than by endocytosis, prevents transcellular oral absorption and causes sequestration to bone. Therefore, prodrug strategies have been developed to temporarily disguise the charges of these compounds. This review examines the various systems that have been used to mask the phosphorus-containing moieties of pyrophosphates and bisphosphonates and also illustrates the utility of such prodrugs.

Bisphosphonates, Old Friends of Bones and New Trends in Clinics

Bisphosphonates, used for a long time in osteoporosis management, are currently the target of intensive research, from pre-formulation studies to more advanced stages of clinical practice. This review presents an overview of the contributions of this family of compounds to human health, starting with the chemistry and clinical uses of bisphosphonates. Following this, their pharmacology is described, highlighting administration-borne handicaps and undesirable effects. The last three sections of the review describe the research efforts that seek to curb delivery-related issues and expand bisphosphonate use. Innovative routes and strategies of administration, such as nano-encapsulation for oral intake or injectable cements for local or in-bone delivery are presented, as well as the latest results of case studies or preclinical studies proposing new therapeutic indications for the clinically approved bisphosphonates. Finally, a selection of anti-infectious bisphosphonate new drug candidates is shown, with focus on the molecules reported in the last two decades.

Design and Biological Evaluation of Delivery Systems Containing Bisphosphonates

Bisphosphonates have found application in the treatment of reoccurrence of bone diseases, breast cancer, etc. They have also been found to exhibit antimicrobial, anticancer and antimalarial activities. However, they suffer from pharmacological deficiencies such as toxicity, poor bioavailability and low intestinal adsorption. These shortcomings have resulted in several researchers developing delivery systems that can enhance their overall therapeutic effectiveness. This review provides a detailed overview of the published studies on delivery systems designed for the delivery of bisphosphonates and the corresponding in vitro/in vivo results.

Physiologic Activity of Bisphosphonates – Recent Advances

Background:

Bisphosphonates are drugs commonly used for the medication and prevention of diseases caused by decreased mineral density. Despite such important medicinal use, they display a variety of physiologic activities, which make them promising anti-cancer, anti-protozoal, antibacterial and antiviral agents.

Objective:

To review physiological activity of bisphosphonates with special emphasis on their ongoing and potential applications in medicine and agriculture.

Method:

Critical review of recent literature data.

Results:

Comprehensive review of activities revealed by bisphosphonates.

Conclusion:

although bisphosphonates are mostly recognized by their profound effects on bone physiology their medicinal potential has not been fully evaluated yet. Literature data considering enzyme inhibition suggest possibilities of far more wide application of these compounds. These applications are, however, limited by their low bioavailability and therefore intensive search for new chemical entities overcoming this shortage are carried out.

Structural requirements for bisphosphonate binding on hydroxyapatite: NMR study of bisphosphonate partial esters

Eighteen different bisphosphonates, including four clinically used bisphosphonate acids and their phosphoesters, were studied to evaluate how the bisphosphonate structure affects binding to bone. Bisphosphonates with weak bone affinity, such as clodronate, could not bind to hydroxyapatite after the addition of one ester group. Medronate retained its ability to bind after the addition of one ester group, and hydroxy-bisphosphonates could bind even after the addition of two ester groups. Thus, several bisphosphonate esters are clearly bone binding compounds. The following conclusions about bisphosphonate binding emerge: (1) a hydroxyl group in the geminal carbon takes part in the binding process and increases the bisphosphonate's ability to bind to bone; (2) the bisphosphonate's ability to bind decreases when the amount of ester groups increases; and (3) the location of the ester groups affects the bisphosphonate's binding ability.

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.

Sequential posttranslational modifications program FEN1 degradation during cell-cycle progression

We propose that cell-cycle-dependent timing of FEN1 nuclease activity is essential for cell-cycle progression and the maintenance of genome stability. After DNA replication is complete at the exit point of the S phase, removal of excess FEN1 may be crucial. Here, we report a mechanism that controls the programmed degradation of FEN1 via a sequential cascade of posttranslational modifications. We found that FEN1 phosphorylation stimulated its SUMOylation, which in turn stimulated its ubiquitination and ultimately led to its degradation via the proteasome pathway. Mutations or inhibitors that blocked the modification at any step in this pathway suppressed FEN1 degradation. Critically, the presence of SUMOylation- or ubiquitination-defective, nondegradable FEN1 mutant protein caused accumulation of Cyclin B, delays in the G1 and G2/M phases, and polyploidy. These findings may represent a newly identified regulatory mechanism used by cells to ensure precise cell-cycle progression and to prevent transformation.

Synthesis and biological evaluation of a new polymeric conjugate and nanocarrier with osteotropic properties

Bone-seeking (osteotropic) drug delivery systems (ODDS) represent an interesting solution for targeting different types of drugs to the bones. In particular, anticancer and antibacterial agents could take advantage of such therapeutic strategy. We have recently developed an innovative approach to this aim: a new osteotropic biomaterial was prepared, based on the conjugation of a poly(lactide-co-glycolide) (PLGA) with the bisphosphonate drug alendronate (PLGA-ALE); its hemo- and cytocompatibility were verified. Starting with this copolymer, an osteotropic nanoparticle system (NP) was produced for the targeted delivery of antineoplastic drugs to osteolytic bone metastases; in particular, doxorubicin was tested as a model drug. The in vitro and in vivo results of the new ODDS are validated in this article. All the experimental data confirmed that the drug retained its activity after loading in the PLGA-ALE NP; they can be thus considered a new promising strategy for active targeting of drugs to bone tissues in different pathological situations.

[Development of delivery system of bisphosphonates for the treatment of osteoporosis]

Bisphosphonates are carbon-substituted pyrophosphate (PCP) analogues that exhibit high affinity to hydroxylapatite and inhibit bone resorption after their administration. They are widely used as the first-choice drug for the treatment and prevention of bone diseases, including Paget's disease, hypercalcemia of malignancy, and osteoporosis. However, the oral bioavailability of bisphosphonates is quite low (1-2%). In addition, the oral administration of bisphosphonates has been associated with mucosal damage, including gastritis, gastric ulcer, and erosive esophagitis. Therefore, it is highly desirable to develop new delivery systems that improve their bioavailability and safety. In this review, recent challenges in the developments of novel delivery system of bisphosphonates are summarized. Then, future developments of delivery system of bisphosphonates are also discussed in order to improve their therapeutic efficacy and safety in the treatment of bone diseases.

A novel biomaterial for osteotropic drug nanocarriers: synthesis and biocompatibility evaluation of a PLGA–ALE conjugate

Background & aims: Osteotropic drug-delivery systems have been proposed as a means to provide drugs with affinity to bone tissues. Drugs or proteins have been linked chemically to bone-seeking agents, such as bisphosphonates (BPs); alternatively, drug-loaded nanoparticles have been used to target specific tissues, such as tumor areas. In our current research, these approaches were merged by synthesizing a novel bone-seeking polymer conjugate, from which targetable nanoparticles can be produced. Materials & methods: An amino-BP, alendronate (ALE) was bound covalently to a biodegradable polymer, poly(lactic-co-glycolide) (PLGA), containing a free end carboxylic group. Blood compatibility and cytotoxicity were assessed in vitro. Results & discussion: By a classical solvent-evaporation method, nanoparticles with a mean size of 200–300 nm were prepared from the conjugate; sterilization was achieved by γ-irradiation, confirming their potential as injectable drug nanocarriers. Owing to the presence of the BP residue, PLGA–ALE nanoparticles were adsorbed onto hydroxyapatite to a higher extent than pure PLGA nanoparticles. The PLGA–ALE conjugate did not induce either hemolysis or alterations of the plasmatic phase of coagulation, or cytotoxic effects on endothelial cells and trabecular osteoblasts. Conclusion: The prepared conjugate represents a novel biomaterial that is able to provide nanoparticles, which can be further loaded with drugs, such as anticancer agents, and addressed to osteolytic or other bone diseases.

Preparation and Evaluation of Floating Risedronate Sodium–Gelucire® 43/01 Formulations

Single and multi-unit floating matrices of risedronate sodium were prepared using Gelucire 43/01 by melt solidification and melt granulation technique, respectively. The controlled release floating matrices were evaluated for in vitro and in vivo floating ability and in vitro drug release. Effect of aging on Gelucire 43/01 was evaluated by hot stage microscopy (HSM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), in vitro floating ability, and in vitro drug release. Multi-unit system obtained has shown initial burst release, which was suppressed in single unit system. Both single- as well as multi-unit systems showed increase in rate of drug release on aging due to changes in the properties of the Gelucire 43/01. Multi-unit matrices obtained by melt granulation were relatively easier for scale up and advantageous if the initial burst release does not cause any significant clinical adversity.

Hydrophilic Anilinogeranyl Diphosphate Prenyl Analogues Are Ras Function Inhibitors†

Sequential processing of H-Ras by protein farnesyl transferase (FTase), Ras converting enzyme (Rce1), and protein-S-isoprenylcysteine O-methyltransferase (Icmt) to give H-Ras C-terminal farnesyl-S-cysteine methyl ester is required for appropriate H-Ras membrane localization and function, including activation of the mitogen-activated protein kinase (MAPK) cascade. We employed a Xenopus laevis oocyte whole-cell model system to examine whether anilinogeranyl diphosphate analogues of similar shape and size, but with a hydrophobicity different from that of the FTase substrate farnesyl diphosphate (FPP), could ablate biological function of H-Ras. Analysis of oocyte maturation kinetics following microinjection of in vitro analogue-modified H-Ras into isoprenoid-depleted oocytes revealed that analogues with a hydrophobicity near that of FPP supported H-Ras biological function, while the analogues p-nitroanilinogeranyl diphosphate (p-NO2-AGPP), p-cyanoanilinogeranyl diphosphate (p-CN-AGPP), and isoxazolaminogeranyl diphosphate (Isox-GPP) with hydrophobicities 2-5 orders of magnitude lower than that of FPP did not. We found that although H-Ras modified with FPP analogues p-NO2-AGPP, p-CN-AGPP, and Isox-GPP was an efficient substrate for C-terminal postprenylation processing by Rce1 and Icmt, co-injection of H-Ras with analogues p-NO2-AGPP, p-CN-AGPP, or Isox-GPP could not activate MAPK. We propose that H-Ras biological function requires a minimum lipophilicity of the prenyl group to allow important interactions downstream of the C-terminal processed H-Ras protein. The hydrophilic FPP analogues p-NO2-AGPP, p-CN-AGPP, and Isox-GPP are H-Ras function inhibitors (RFIs) and serve as lead compounds for a unique class of potential anticancer therapeutics.

Synthesis of novel (1-alkanoyloxy-4-alkanoylaminobutylidene)-1,1-bisphosphonic acid derivatives

A novel strategy for the synthesis of (1-alkanoyloxy-4-alkanoylaminobutylidene)-1,1-bisphosphonic acid derivatives (1a-d) via (1-hydroxy-4-alkanoylaminobutylidene)-1,1-bisphosphonic acid derivatives (2a-d), starting from alendronate has been developed with reasonable 51–77% overall yields. Intermediate products, (1-hydroxy-4-alkanoylaminobutylidene)-1,1-bisphosphonic acid derivatives (2a-d), were prepared in water with reasonable to high yields (52–94%).

Radical reaction of sodium hypophosphite with terminal alkynes: synthesis of 1,1-bis-H-phosphinates

[reaction: see text] The room-temperature radical addition of sodium hypophosphite to terminal alkynes produces the previously unknown 1-alkyl-1,1-bis-H-phosphinates in moderate yield. The reaction is initiated by R3B and air and proceeds under mild conditions in an open container. The bissodium salts precipitate spontaneously from the reaction mixtures, thus providing a simple purification procedure and the opportunity for multigram synthesis. The 1,1-bis-H-phosphinate products are novel precursors of the biologically important 1,1-bisphosphonates.

Synthesis of acyloxymethyl ester prodrugs of the transferable protein farnesyl transferase substrate farnesyl methylenediphosphonate

Three isoprenoid diphosphate analogues of farnesyl diphosphate (FPP) where the diphosphate has been replaced by methylene diphosphonate and the negative charges masked by frangible pivaloyloxymethyl (POM) esters were prepared. Farnesyl methylenediphosphonate is a sub-micromolar substrate for protein farnesyl transferase. The tripivaloyloxymethyl esters of isoprenoid methylenediphosphonate have significantly increased lipophilicity and may act as important farnesyl diphosphate prodrugs.

Efficient synthesis of phosphorylated prodrugs with bis(POM)-phosphoryl chloride

An efficient method for the synthesis of phosphorylated prodrugs is described. The preparation of various bis-pivaloyloxymethyl (POM) phosphate triesters was accomplished in moderate to good yields with the use of bis(POM) phosphoryl chloride under mild conditions.

Synthesis, in vitro evaluation, and antileishmanial activity of water-soluble prodrugs of buparvaquone

Water-soluble phosphate prodrugs of buparvaquone (1), containing a hydroxynaphthoquinone structure, were synthesized and evaluated in vitro for improved topical and oral drug delivery against cutaneous and visceral leishmaniasis. The successful prodrug synthesis involved a strong base; e.g., sodium hydride. Buparvaquone-3-phosphate (4a) and 3-phosphonooxymethyl-buparvaquone (4b) prodrugs possessed significantly higher aqueous solubilities (>3.5 mg/mL) than the parent drug (</=0.03 microg/mL) over a pH range of 3.0-7.4. Moreover, 4a and 4b maintained adequate lipophilicity as indicated by distribution coefficients (log D) between 0.5 and 3.0 over this pH range. Both 4a and 4b were also shown to be substrates for alkaline phosphatase in vitro and thus are promising bioreversible prodrugs for the improved topical and oral bioavailability of 1. Buparvaquone and its prodrugs showed nanomolar or low-micromolar ED(50) activity values against species that cause cutaneous leishmaniasis, e.g., L. major, L. amazonensis, L. aethiopica, L. mexicana, and L. panamensis and also L. donovani, which is the causative agent of visceral leishmaniasis. From these results, the human skin permeation of the prodrugs 4a and 4b were studied in vitro. While no buparvaquone permeated across post mortem skin in vitro during 72 h of experiments, both prodrugs 4a and 4b permeated readily through the skin. In addition, 4b easily released the parent drug in human skin homogenate and, therefore, is a promising prodrug candidate to deliver buparvaquone through the skin for the treatment of cutaneous leishmaniasis.

SYNTHESIS OF SYMMETRICAL METHYLENEBIS(ALKYL HYDROGEN PHOSPHONATES) BY SELECTIVE CLEAVAGE OF METHYLENEBIS(DIALKYL PHOSPHONATES) WITH MORPHOLINE

The preparation of partial esters of methylenebisphosphonic acids has been of recent interest due to their potential therapeutic applications. This paper describes a convenient method to prepare symmetrical methylenebis(alkyl hydrogen phosphonates) by the selective cleavage of the corresponding methylenebis(dialkyl phosphonate) with refluxing morpholine. The effects of structural variations on the amine as well as the substrate have been investigated to understand the scope and limitations of this reaction. A superior approach to hindered bisphosphonic acid esters involves the cleavage of their dimethyl esters, 4, using morpholine. This method is also useful to access a number of C-alkyl dialkyl methylenebisphosphonic acids such as 6. This study clearly shows that cleavage with morpholine is convenient, inexpensive, and allows for the preparation of a variety of P,P'-disubstituted partial esters in good yields and high purity.

Effects of various absorption enhancers on transport of clodronate through Caco-2 cells

The major disadvantage concerning clinical use of bishosphonate drugs, like clodronate, is their poor and variable absorption after oral administration. The objective of this study was to assess the effects of four different absorption enhancers-palmitoyl carnitine chloride (PCC), N-trimethyl chitosan chloride (TMC), sodium caprate (C10), and ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA)-on the transport of clodronate using Caco-2 cell culture model. The transport experiments were performed in a normal (1.3mM) and in a minimum-calcium concentration (apically calcium-free medium and basolaterally 100 microM calcium concentration). In the normal calcium concentration, a strong enhancement in clodronate permeation was observed with the enhancers: EGTA (2.5mM), TMC (1.5% w/v), and PCC (0.2mM) increased the transport of 1mM clodronate 190-, 20-, and 10-fold, respectively, and the transport of 10mM clodronate 130-, 70-, and 35-fold. In the minimum-calcium concentration, the effects of the absorption enhancers on the transport of clodronate were not so potent: TMC, PCC, and EGTA caused 2- to 20-fold enhancement in clodronate permeation whereas C10 (10mM) was without any effect. According to the results, the permeation of clodronate through Caco-2 cells could be significantly promoted by the absorption enhancers, which cause widening of the tight junctions and, thus, increase the permeability of the paracellular route.

Cellular uptake and metabolism of clodronate and its derivatives in Caco-2 cells: a possible correlation with bisphosphonate-induced gastrointestinal side-effects

PURPOSE

To investigate possible reasons for the low frequency of GI side-effects of clodronate, even though clodronate is known to be metabolised into a cytotoxic nucleotide analogue (AppCCl(2)p) by many cell types. The effects of some lipophilic prodrugs of clodronate were also studied.

METHODS

The effects of clodronate and its lipophilic derivatives on the proliferation and viability of Caco-2 cells were examined using an MTT assay. The intracellular uptake of 14C-clodronate and the accumulation of a clodronate metabolite (AppCCl(2)p) in Caco-2 cells were evaluated using ion-pairing HPLC-ESI-MS.

RESULTS

Clodronate had little effect on growth of proliferating, or the viability of confluent, Caco-2 cells. The uptake of clodronate by Caco-2 cells was only about 0.04% of total clodronate. The potentially cytotoxic clodronate metabolite, AppCCl(2)p, was detected in Caco-2 cell extracts after 3 h of exposure. Dianhydride- and triPOM-clodronate were metabolised to AppCCl(2)p more efficiently and also affected the viability of Caco-2 cells more than clodronate.

CONCLUSIONS

Clodronate appears to be metabolised into a cytotoxic ATP-analogue (AppCCl(2)p) by any cell type capable of internalising the drug. However, the cytotoxicity depends on the degree of uptake of clodronate. Due to the very low initial uptake of clodronate by epithelial Caco-2 cells, they do not accumulate sufficient intracellular concentrations of AppCCl(2)p to affect cell function. This explains the low frequency of gastrointestinal side-effects caused by oral clodronate therapy.

Pivalase catalytic antibodies: towards abzymatic activation of prodrugs

Screening of monoclonal-antibody libraries generated against the tert-butyl phosphonate hapten 2 and the chloromethyl phosphonate hapten 3 with pivaloyloxymethyl-umbelliferone 1 as a fluorogenic substrate led to the isolation of eleven catalytic antibodies with rate accelerations around kcat/ kuncat = 10(3). The antibodies are not inhibited by the product and accept different acyloxymethyl derivatives of acidic phenols as substrates. The highest activity was found for the bulky, chemically less-reactive pivaloyloxymethyl group: there is no activity with acetoxymethyl or acetyl esters. This difference might reflect the preference of the immune system for hydrophobic interactions in binding and catalysis. Pivalase catalytic antibodies might be useful for activating orally available pivaloyloxymethyl prodrugs.

Bisphosphonate prodrugs: synthesis and in vitro evaluation of alkyl and acyloxymethyl esters of etidronic acid as bioreversible prodrugs of etidronate

The synthesis and preliminary evaluation of novel alkyl and acyloxymethyl esters of etidronic acid as etidronate prodrugs is reported. Tetramethyl ester of etidronic acid was found be isomerized at pH 7.4 and P-C-P bridge was rearranged to P-C-O-P. This unwanted process was prevented via acylation of the bridging carbon's alcohol group. Acylation showed to be stable if one or more phosphonic OH- groups were substituted. However, when none of the phosphonic OH- groups were substituted, the acylation was chemically hydrolysed and the parent drug was released. This finding was successfully applied in the design of tetrapivaloyloxymethyl ester of acetylated etidronic acid which released etidronic acid via enzymatic (first step) and chemical (second step) hydrolysis in liver homogenate. However, the corresponding tri-substituted pivaloyloxymethyl ester having adequate water-solubility and lipophilicity (logP(app) 0.6 at pH 7.4), is probably the most potential prodrug candidate reported to enhance the oral bioavailability of etidronate.