Osteotropic drug delivery system (ODDS) based on bisphosphonic prodrug. I: synthesis and in vivo characterization of osteotropic carboxyfluorescein

Dendrimer-based micelles with highly potent targeting to sites of active bone turnover for the treatment of bone metastasis

Bone-drug targeting therapies using nanoparticles based on targeting ligands remain challenging due to their uptake clearance at non-target sites such as the liver, kidney, and spleen. Furthermore, the distribution sites of nanoparticles in bones have not been fully investigated, thus halting the development of more effective bone metastasis treatment strategies. In this study, we developed nanoparticles self-assembled from cholesterol-terminated, polyethylene glycol-conjugated, aspartic acid (Asp)-modified polyamidoamine dendrimer (Asp-PAMAM-Micelles) with targeting to active bone turnover sites associated with bone metastasis pathogenesis. On analysis through whole-body single photon emission computed tomography/computed tomography (SPECT/CT) imaging, ¹¹¹In-Asp-PAMAM-Micelles showed high specificity to active bone turnover sites (especially the joints in the lower limbs, shoulder, and pelvis) after intravenous injection in mice. The lower limb bone uptake clearance for ¹¹¹In-Asp-PAMAM-Micelles encapsulating paclitaxel (PTX) was 3.5-fold higher than that for ¹¹¹In-unmodified PAMAM-Micelles (PTX). ³H-PTX encapsulated Asp-PAMAM-Micelles effectively accumulated in the lower limb bones in a similar manner as the ¹¹¹In-Asp-PAMAM-Micelles (PTX). In a bone metastatic tumor mouse model, the tumor growth in the lower limb bones was significantly inhibited by injection of Asp-PAMAM-Micelles (PTX) compared to unmodified PAMAM-Micelles (PTX). Our results demonstrate that Asp-PAMAM-Micelles are sophisticated drug delivery systems for highly potent targeting to active bone turnover sites.

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.

Bone selective effect of an estradiol conjugate with a novel tetracycline-derived bone-targeting agent

In this study a novel bone-targeting agent containing elements of the tricarbonylmethane system of ring A of tetracycline was developed and was shown to bind to the mineral constituent of bone, hydroxyapatite. Conjugation of this bone-targeting agent to estradiol resulted in a bone-targeted estrogen (BTE(2)-A1) with an enhanced ability to bind to hydroxyapatite. In an ovariectomized rat model of osteoporosis a partial separation of the skeletal effects of estradiol from the uterine effects was observed following subcutaneous administration of BTE(2)-A1. This novel bone-targeting estradiol delivery system has the potential to improve the safety profile of estradiol in the treatment of osteoporosis.

Linking bisphosphonates to the free amino groups in fluoroquinolones: preparation of osteotropic prodrugs for the prevention of osteomyelitis

Osteomyelitis is an infection located in bone and a notoriously difficult disease to manage, requiring frequent and heavy doses of systemically administered antibiotics. Targeting antibiotics to the bone after systemic administration may provide both greater efficacy of treatment and less frequent administration. By taking advantage of the affinity of the bisphosphonate group for bone mineral, we have prepared a set of 13 bisphosphonated antibacterial prodrugs based on eight different linkers tethered to the free amino functionality on fluoroquinolone antibiotics. While all but one of the prodrugs were shown in vitro to be effective and rapid bone binders (over 90% in 1 h), only eight of them demonstrated the capacity to significantly regenerate the parent drug. In a rat model of the disease, a selected group of agents demonstrated their ability to prevent osteomyelitis when used in circumstances under which the parent drug had already been cleared and is thus inactive.

Bisphosphonated fluoroquinolone esters as osteotropic prodrugs for the prevention of osteomyelitis

Osteomyelitis is a difficult to treat bacterial infection of the bone. Delivering antibacterial agents to the bone may overcome the difficulties in treating this illness by effectively concentrating the antibiotic at the site of infection and by limiting the toxicity that may result from systemic exposure to the large doses conventionally used. Using bisphosphonates as osteophilic functional groups, different forms of fluoroquinolone esters were synthesized and evaluated for their ability to bind bone and to release the parent antibacterial agent. Bisphosphonated glycolamide fluoroquinolone esters were found to present a profile consistent with effective and rapid bone binding and efficient release of the active drug moiety. They were assessed for their ability to prevent bone infection in vivo and were found to be effective when the free fluoroquinolones were not.

Statins and osteoporosis: new role for old drugs

Osteoporosis is the most common bone disease, affecting millions of people worldwide and leading to significant morbidity and high expenditure. Most of the current therapies available for its treatment are limited to the prevention or slowing down of bone loss rather than enhancing bone formation. Recent discovery of statins (HMG-CoA reductase inhibitors) as bone anabolic agents has spurred a great deal of interest among both basic and clinical bone researchers. In-vitro and some animal studies suggest that statins increase the bone mass by enhancing bone morphogenetic protein-2 (BMP-2)-mediated osteoblast expression. Although a limited number of case-control studies suggest that statins may have the potential to reduce the risk of fractures by increasing bone formation, other studies have failed to show a benefit in fracture reduction. Randomized, controlled clinical trials are needed to resolve this conflict. One possible reason for the discrepancy in the results of preclinical, as well as clinical, studies is the liver-specific nature of statins. Considering their high liver specificity and low oral bioavailability, distribution of statins to the bone microenvironment in optimum concentration is questionable. To unravel their exact mechanism and confirm beneficial action on bone, statins should reach the bone microenvironment in optimum concentration. Dose optimization and use of novel controlled drug delivery systems may help in increasing the bioavailability and distribution of statins to the bone microenvironment. Discovery of bone-specific statins or their bone-targeted delivery offers great potential in the treatment of osteoporosis. In this review, we have summarized various preclinical and clinical studies of statins and their action on bone. We have also discussed the possible mechanism of action of statins on bone. Finally, the role of drug delivery systems in confirming and assessing the actual potential of statins as anti-osteoporotic agents is highlighted.

Cathepsin K inhibitor–polymer conjugates: potential drugs for the treatment of osteoporosis and rheumatoid arthritis

The role of the newly discovered cysteine protease, cathepsin K, in osteoporosis and rheumatoid arthritis is reviewed. The current development of cathepsin K inhibitors and their targeted delivery using synthetic polymer carriers are discussed. Future challenges and possible strategies to improve these delivery systems are addressed.

Bone as an effect compartment : models for uptake and release of drugs

"Bone-seeking agents" are drugs characterised by high affinity for bone, and are disposed in bone for prolonged periods of time while maintaining remarkably low systemic concentrations. As a consequence, the bone becomes a reservoir for bone-seeking agents, and a site of both desirable and adverse effects, depending on the pharmacological activities of the specific agent. For some agents, significant systemic effects may also be produced following their prolonged release from bone, a process that is governed mostly by the rate of bone remodelling. This review covers the pharmacokinetic and pharmacodynamic features of bone-seeking agents with different pharmacological properties, including drugs (bisphosphonates, drug-bisphosphonate conjugates, radiopharmaceuticals and fluoride), bone markers (tetracycline, bone imaging agents) and toxins (lead, chromium, aluminium). In addition, drugs that do not possess bone-seeking properties but are used for therapy of bone diseases (such as antibacterials for treatment of osteomyelitis) are discussed, along with targeting of these drugs to the bone by conjugation to bone-seeking agents, local delivery systems, and other approaches. The pharmacokinetic and pharmacodynamic behaviour of bone-seeking agents is extremely complex due to heterogeneity in bone morphology and physiology. This complexity, accompanied by difficulties in human bone research caused by ethical and other limitations, gave rise to modelling approaches to study bone drug disposition. This review describes the pharmacokinetic models that have been proposed to describe the pharmacokinetic behaviour of bone-seeking agents and predict bone concentrations of these agents for different doses and patient populations. Models of different types (compartmental and physiologically based) and of different complexity have been applied, but their relevance to drug effects in the bone tissue is limited since they describe the behaviour of the "average" drug molecule. Understanding of the cellular and molecular processes responsible for the heterogeneity of bone tissue will provide better comprehension of the influence of microenvironment on drug bone disposition and the resulting pharmacological response.

Bone-Specific Drug Delivery Systems

Despite several decades of progress, bone-specific delivery is still limited by the unique anatomical features of bone, which mainly consists of inorganic hydroxyapatite. A practical approach to this problem is to produce targeted drugs that have a high affinity for hydroxyapatite. Bisphosphonates are a class of synthetic compounds structurally related to pyrophosphate. Bisphosphonates rapidly localise on the bone surface after being administered either intravenously or orally, since the P-C-P portion of the bisphosphonate structure has high affinity for hydroxyapatite. Therefore, bisphosphonate modification might be a promising method for targeting drugs selectively to the bone. Bisphosphonate-conjugated drugs are hydrophilic and highly water-soluble due to the acidic nature of the bisphosphonate moiety at physiological pH, and therefore they hardly permeate through the biological membrane of soft tissues. These physicochemical changes also reduce the intrinsic susceptibility of the drug to metabolism, promoting urinary or biliary excretion as unchanged drug. All these physicochemical and pharmacokinetic alterations contribute to the exceptional skeletal disposition of bisphosphonate-conjugated drugs. Bisphosphonate conjugation is based on chemical modification of the targeting molecule, and therapeutically optimised bisphosphonate derivatives have to be custom-developed on a case-by-case basis. The bisphosphonate moiety is usually coupled with the targeting drug through a specific linkage. The high affinity of bisphosphonate conjugates for the bone is not simply dependent on the bisphosphonate moiety but on the resultant molecule as a whole, including the linker and the linked drug. Lipophilicity (represented as log P) appears to be an appropriate index for predicting the osteotropic properties of bisphosphonate derivatives. Several strategies using bisphosphonate-conjugated drugs have been investigated at a laboratory level with the aim of obtaining therapeutically optimised treatments for conditions such as osteoporosis, osteoarthritis and bone cancer. In each case, the intention is to achieve prolonged local exposure to high concentrations of the targeting drug, thereby improving therapeutic index by enhancing pharmacological efficacy and minimising systemic adverse effects. Although most examples of bone-specific drug delivery via bone-seeking agents still remain in preclinical studies, several phosphonate-coupled radiopharmaceuticals, such as samarium-153 complexed to tetraphosphonate, are expected to be an effective pain palliation therapies for metastatic bone cancer and are currently being developed in clinical trials. Furthermore, recent reports on bisphosphonate-modified proteins have illustrated the feasibility of bone-specific delivery of biologically active protein drugs, such as cytokines and growth factors.

Dose-dependent pharmacokinetics and disposition of bisphosphonic prodrug of diclofenac based on osteotropic drug delivery system (ODDS)

Rat pharmacokinetics and in vivo disposition of a novel bisphosphonic prodrug of diclofenac (DIC-BP), synthesized with the aim of osteotropic delivery of diclofenac, were determined at whole body, organ and cellular levels in a dose range 0.32-10mg/kg. With an increase in injected dose, total body clearance was decreased while the distribution volume at steady state (V(dss)) was reduced and plasma half-life was prolonged. Over 50% of a dose of DIC-BP was selectively transported into osseous tissues after intravenous injection into rats at doses up to 1mg/kg. As dose increased, the skeletal distribution decreased with hepatic and splenic accumulations increasing. The intrahepatic distribution at 10mg/kg revealed that liver macrophages play a significant role in hepatic uptake of DIC-BP. This is consistent with general arguments that bisphosphonates themselves cannot distribute in soft tissues, but are taken up by the reticuloendothelial system as foreign substances when they form large complexes or aggregate with endogenous metals in plasma. Therefore, to optimize the osteotropic delivery of diclofenac via a bisphosphonic prodrug, the dosage regimen should be such that plasma concentration of DIC-BP is maintained at a level lower than that required for precipitate formation of complexes, similar to the usage of other bisphosphonates.

Inhibition of cathepsin K with lysosomotropic macromolecular inhibitors

Cathepsin K is the major enzyme responsible for the degradation of the protein matrix of bone and probably for the destruction of articular cartilage in rheumatoid arthritis joints. These processes occur mainly in the resorption lacuna and within the lysosomal compartment. Here, we have designed, synthesized, and evaluated new lysosomotropic (water-soluble) polymer-cathepsin K inhibitor conjugates. In particular, we characterized the relationship between conjugate structures and their activity to inhibit cathepsins K, B, L, and papain. A potent selective cathepsin K inhibitor, 1,5-bis(N-benzyloxycarbonylleucyl)carbohydrazide, was modified to 1-(N-benzyloxycarbonylleucyl)-5-(phenylalanylleucyl)carbohydrazide (I) to facilitate polymer conjugation. It was conjugated to the polymer chain termini of two water-soluble polymers [alpha-methoxy poly(ethylene glycol), abbreviated as mPEG-I; semitelechelic poly[N-(2-hydroxypropyl)methacrylamide], abbreviated as ST-PHPMA-I]. The conjugation of inhibitor I to N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer side chains was accomplished via either a Gly-Gly spacer (PHPMA-GG-I) or with no spacer between I and the copolymer backbone (PHPMA-I). Kinetic analysis revealed that free inhibitor I possessed an apparent second-order rate constant against cathepsin K (k(obs)/[I] = 1.3 x 10(6) M(-1) s(-1)) similar to that of unmodified 1,5-bis(Cbz-Leu) carbohydrazide, while I conjugated to the chain termini of mPEG and ST-PHPMA-COOH had slightly lower values (about 5 x 10(5) M(-1) s(-1)). The k(obs)/[I] values for I attached to the side chains of HPMA copolymers (PHPMA-GG-I and PHPMA-I) were about 3 x 10(4) M(-1) s(-1). When tested against cathepsin L, inhibitor I and all its polymer conjugates produced k(obs)/[I] values 1-2 orders of magnitude less than those determined for cathepsin K, while for cathepsin B and papain, the values were 2-4 orders of magnitude lower. The ability of mPEG-I and ST-PHPMA-I to inhibit cathepsin K activity in synovial fibroblasts was also evaluated. Both polymer-bound inhibitors were internalized by endocytosis and were ultimately trafficked to the lysosomal compartment. ST-PHPMA-I was internalized faster than mPEG-I. The inhibitory activity in the synovial fibroblast assay correlated with the rate of internalization.

In vivo near-infrared fluorescence imaging of osteoblastic activity

In vertebrates, the development and integrity of the skeleton requires hydroxyapatite (HA) deposition by osteoblasts. HA deposition is also a marker of, or a participant in, processes as diverse as cancer and atherosclerosis. At present, sites of osteoblastic activity can only be imaged in vivo using gamma-emitting radioisotopes. The scan times required are long, and the resultant radioscintigraphic images suffer from relatively low resolution. We have synthesized a near-infrared (NIR) fluorescent bisphosphonate derivative that exhibits rapid and specific binding to HA in vitro and in vivo. We demonstrate NIR light-based detection of osteoblastic activity in the living animal, and discuss how this technology can be used to study skeletal development, osteoblastic metastasis, coronary atherosclerosis, and other human diseases.

Relationship between physicochemical and osteotropic properties of bisphosphonic derivatives: rational design for osteotropic drug delivery system (ODDS)

PURPOSE

The objective of this investigation is to develop a rational design of Osteotropic Drug Delivery System (ODDS), which we have proposed as a novel method for drug delivery to the skeleton via bisphosphonic prodrug, based on the relationship between physicochemical and pharmacokinetic properties of bisphosphonates.

METHODS

The theoretical octanol/water partition coefficients (clog P) of 13 bisphosphonates were calculated by computer software, CLOGP ver. 3.05 (Daylight C.I.S., Inc. Irvine, CA) and related to pharmacokinetic or osteotropic parameters after intravenous injection into rats. On the other hand, to optimize ODDS of diclofenac (DIC-BP), the effects of doses or infusion rates on the in vivo disposition were investigated in relation to solubility product value (Ksp) of DIC-BP-calcium complex.

RESULTS

Clog P had good correlations with total plasma clearance, apparent distribution volume and the fraction dose delivered to the whole skeleton after bolus injection into rats (r = -0.868 approximately -0.914). The targetability of bisphosphonates to the skeleton was linearly decreased with an increase in clog P value and the more hydrophilic bisphosphonates were suitable for ODDS in bolus administration. On the other hand, DIC-BP, a relatively lipophilic bisphosphonate, was effectively and selectively delivered to the skeleton only when administered as a slow infusion to keep plasma concentration lower than that calculated from Ksp value where DIC-BP could precipitate with calcium in the plasma circulation.

CONCLUSIONS

Our results suggest the possibility of a rational design of ODDS via bisphosphonic prodrugs, after consideration of compound lipophilicity and precipitability of bisphosphonate-calcium complex.

Novel drug delivery system to bone using acidic oligopeptide: pharmacokinetic characteristics and pharmacological potential

We synthesized fifteen oligopeptides consisting of Asp or Glu conjugated with a fluorescent probe, 9- fluorenylmethylchloroformate (Fmoc). In the in vitro binding assay to putative hydroxyapatite (HA), the affinities of these conjugates depended only on the number of amino acid residues, not on their optical characters (L or D) or their species (Asp or Glu). In an in vivo experiment involving a single i.v. injection of Fmoc-D-Asp oligopeptides into mice, peptides consisting of over six Asp residues were selectively distributed to the bone. Then, we synthesized estradiol-17 beta-succinate-(L-Asp)6 [E2-(L-Asp)6] and studied its pharmacokinetic characteristics and its antiosteoporotic effects on ovariectomized (OVX) mice. Although the distribution volume of E2-(L-Asp)6 was significantly smaller than that of E2, E2-(L-Asp)6 was selectively distributed in the bone after i.v. injection and gradually decreased during 7 days. E2-(L-Asp)6 effectively prevented OVX-induced bone loss, without altering the uterine weight, in the dosage range of 0.11 to 1.1 mumol/kg once a week, while E2 increased both the bone mineral density and uterine weight at 0.37 mumol/kg every third day. The results suggest that acidic oligopeptide may be useful for drug delivery to bone and E2-(L-Asp)6 is a good candidate as an anti-osteoporosis drug without the adverse side effects of E2.

Bone-specific delivery and sustained release of diclofenac, a non-steroidal anti-inflammatory drug, via bisphosphonic prodrug based on the Osteotropic Drug Delivery System (ODDS)

We have newly synthesized osteotropic diclofenac with bisphosphonic moiety (DIC-BP) based on the concept of Osteotropic Drug Delivery System (ODDS) and investigated its potency of site-specific and controlled delivery of diclofenac to the bone in rats. After intravenous injection into rats, DIC-BP was predominantly distributed in the skeleton. DIC-BP once incorporated in the bone was gradually eliminated (t(1/2)=9.7 days), releasing diclofenac into the bone compartment. As a result, the bone concentration of regenerated diclofenac was apparently constant over 28 days. Furthermore, we evaluated the anti-inflammatory effects of DIC-BP compared with diclofenac (sodium salt) in adjuvant-induced arthritic rats. The mean effective doses (ED(50)) were 0.55 mg/kg and 1.3 mg/kg for daily oral administration of diclofenac and weekly intravenous injection of DIC-BP, respectively. Considering the frequency of medication of 17 times for diclofenac and 4 times for DIC-BP in the experimental period, ED(50) was corrected to 9.4 and 5.2 mg/kg (per experimental period) for diclofenac and DIC-BP, respectively. Moreover, DIC-BP exhibited no side effects of gastrointestinal damage, typical of non-steroidal anti-inflammatory drugs. Thus, ODDS of diclofenac shows promise as an approach for highly potent and non-toxic therapy of diclofenac, with less frequent medication.

Administration routes and delivery systems of bisphosphonates for the treatment of bone resorption

Geminal bisphosphonates (BPs) are a class of drugs considered to be stable analogs of pyrophosphate (P-O-P), a physiological regulator of calcification and bone resorption. A number of BPs have been approved for clinical use in Paget's disease, hypercalcemia of malignancy, and osteoporosis. The major disadvantage of the clinically utilized BPs is their poor oral absorption from the GI tract, typically less than 1% is absorbed. In addition, the BPs have been associated with adverse gastrointestinal effects in humans. The challenge for novel drug delivery systems is to achieve improved bioavailability and safety. In the first part of this review, we discuss the bioavailability of BPs, the effect of food on the absorption of BPs, the mechanism of BPs' absorption and the adverse gastrointestinal effects. In the second part of the review, various methods that have been used for improving the bioavailability of BPs are described. Dosage form strategies reviewed include the use of particular formulations for increasing oral absorption as well as decreasing adverse gastrointestinal effects, absorption enhancers, BP compounds and the solubility of their calcium complex/salts, and the prodrug approach. Because of the poor GI absorption, attempts have been made to enhance the bioavailability of BPs by several parenteral routes other than i.v. injections. Description of nasal administration, s.c. and i.m. injections, BP implants and targeted osteotropic delivery systems are reviewed.

Selective drug delivery system to bone: small peptide (Asp)6 conjugation

Targeting a drug on hydroxyapatite (HA) could be a promising way for selective drug delivery to bone, because HA, an inorganic component in hard tissues (bone and teeth), does not exist in soft tissues. Several bone noncollagenous proteins, which bind to HA, have repeating sequences of acidic amino acids in their structures as possible HA-binding sites. Thus, we think that a small peptide of repetitive acidic amino acid could work as a carrier for selective drug delivery to the bone. To test this hypothesis, we conjugated (Asp)6 to fluorescein isothiocyanate (FITC), evaluated its affinity to HA in vitro, and examined its tissue distribution after injection into rats. Although fluorescein itself did not bind to HA, (Asp)6-FITC bound to HA as well as calceine and tetracycline. Twenty-four hours after intravenous injection of (Asp)6-FITC to rats, animals were killed, and ground sections of hard tissues and cryosections of soft tissues were made. Under a confocal laser scanning microscope, clear labeling lines were observed in bones and teeth, whereas no labeling was detected in soft tissues. In the rats administered with fluorescein alone, the fluorescent labeling was detected in neither hard nor soft tissues. Fluorescent analysis of blood, urine, and bones after (Asp)6-FITC administration revealed that biological half-life of FITC in blood was short (60 minutes) and that within 24 h, 95% of the administered FITC was excreted as urine whereas 2% of the FITC accumulated in bones. After subcutaneous administration of (Asp)6-FITC to mice, fluorescent intensity remaining in the femurs was measured periodically. In these mice the biological half-life of FITC in the femur was 14 days. Present results indicate that (Asp)6 is effective as a carrier for selective drug delivery to bone.

Osteotropic drug delivery system (ODDS) based on bisphosphonic prodrug. I.v. effects of osteotropic estradiol on bone mineral density and uterine weight in ovariectomized rats

An osteotropic drug delivery system (ODDS) based on the bisphosphonic prodrug was designed for 17beta-estradiol (E2) in order to improve patient compliance in estrogen replacement therapy of postmenopausal osteoporosis. The bisphosphonic prodrug of E2, disodium [17beta-(3 '-hydroxy- 1',3',5'-estratrienyloxy) carbonylpropyl carboxamidomethylene] bisphosphonate (E2-BP) was synthesized and its effects on bone mineral density and uterine weight were investigated in ovariectomized (OVX) rats. E2-BP was injected intravenously once a week (4 injections/experiment), and E2 was administrated orally 5 times a week (20 administrations/experiment). Once a week treatment with 0.1 mg/kg E2-BP significantly restored bone mineral reduction by 61.8% without significantly increasing uterine weight. Similarly, once in 4 weeks treatment with 1.0 mg/kg E2-BP (1 injection/experiment) showed almost the same therapeutic effects. On the other hand, 5 times a week oral treatment with 1.0 mg/kg E2 significantly improved bone mineral density by 90.5%, but increased uterine weight up to 98.2% of that of the sham group. In vitro bone resorption analysis revealed that E2-BP exhibits antiresorptive activity not as a bisphosphonate but as a prodrug of E2. These results demonstrated that E2-BP has the potential to improve patient compliance in estrogen therapy by its minimal adverse effects and less frequent medication.

Physicochemical characterization of bisphosphonic carboxyfluorescein for osteotropic drug delivery

Disodium (fluorescein-6-carbonyloxy)acetoaminomethylene bisphosphonate (CF-BP), a prodrug of 6-carboxy-fluorescein, is efficiently absorbed by the skeleton where it hydrolyses to carboxyfluorescein. An osteotropic drug-delivery system based on this bisphosphonic prodrug has been developed as a novel method for site-specific and controlled delivery of drugs to the bone. In this study the physicochemical properties of the prodrug have been characterized by investigating the affinity of CF-BP for hydroxyapatite and the hydrolysis of the compound to carboxyfluorescein. In the binding study, CF-BP bound very rapidly to hydroxyapatite without degradation and carboxyfluorescein was subsequently gradually released by hydrolysis of bound CF-BP. Hydrolysis of CF-BP in buffer solutions followed pseudo-first-order kinetics, and quantitative regeneration of carboxyfluorescein was observed. In addition, regeneration of carboxyfluorescein from CF-BP was accelerated in the presence of fresh rat plasma. These results suggest that CF-BP has the physicochemical properties required for site-specific and controlled delivery of carboxyfluorescein to bones.

Osteotropic drug delivery system (ODDS) based on bisphosphonic prodrug. III: Pharmacokinetics and targeting characteristics of osteotropic carboxyfluorescein

An osteotropic drug delivery system (ODDS) based on a bisphosphonic prodrug has been developed as a novel method for site-specific and controlled delivery of drugs to the bone. The pharmacokinetics and the targeting efficiency of a bisphosphonic prodrug of carboxyfluorescein (CF), disodium (fluorescein-6-carbonyloxy) acetoaminomethylene bisphosphonate (CF-BP), was investigated in rats. After intravenous injection, CF-BP was rapidly taken up into the skeleton, and subsequently cleared from the bone by hydrolysis of its ester linkage at a half-life of 3.2 days. On the other hand, the bone concentration of regenerated CF gradually increased to reach the maximum at 14 days and slowly decreased up to 56 days. Kinetical analysis revealed that bone tissue acts as a reservoir of regenerated CF to supply the parent compound into the systemic circulation. In contrast with CF-BP, CF injected intravenously showed rapid clearance from the plasma and extremely low bone distribution. Therapeutic availability (TA) and drug targeting index (DTI), which were calculated on the basis of the AUCs for CF in the bone and plasma after injection of CF-BP and CF, were 1551 and 6689, respectively. These results suggest that ODDS has a potential to improve not only apparent potency but also therapeutic index of the drugs which exhibit their pharmacological effects in the bone.