Synthesis of gem-bisphosphonic methotrexate conjugates and their biological response towards Walker's osteosarcoma

Design, Synthesis, and Characterization of HBP-Vectorized Methotrexate Prodrug Molecule 1102-39: Evaluation of In Vitro Cytotoxicity Activity in Cell Culture Models, Preliminary In Vivo Safety and Efficacy Results in Rodents

A novel bone-targeted prodrug, 1102-39, is discussed with the aim of enhancing the therapeutic effects of methotrexate (MTX) within bone tissues while minimizing systemic toxicity. Within the 1102-39 molecule, the central linker part forms a cleavable ester group, with MTX being also linked by a stable imine bond to the specially designed hydroxybisphosphonic (HBP) vector. Synthesized through a convergent approach starting from MTX, this prodrug advantageously modulates MTX's activity by selective esterification of its α-carboxyl group. In vitro tests revealed a 10-fold reduction in cytotoxicity compared to standard MTX, in alignment with prodrug behavior and correlated with gradual MTX release. In vivo in rodents, 1102-39 displayed preliminary encouraging antitumor effects on orthotopic osteosarcoma. Furthermore, various aspects of designing molecules for selective therapy in bone tissue based on bisphosphonate molecules as vectors for delivering active compounds to the bone are discussed. The 1102-39 molecule exhibits strong affinity for hydroxyapatite and a progressive release of MTX in aqueous environments, enhancing the safety and efficacy of bone-specific treatments and enabling sustained activity within bone and bone joints in the therapy of tumor and inflammation.

Zolendronic Acid-Conjugated PLGA Ultrasmall Nanoparticle Loaded with Methotrexate as a Supercarrier for Bone-Targeted Drug Delivery

Drug delivery to deep-seated tissues such as bone has been a major complication till date. This preferential drug delivery is further important in targeting anti-tumour agents to bone metastasis owing to its complexity. The present study involves the formulation of PLGA nanoparticles and conjugation with zolendronic acid-a bisphosphonate which will anchor the nanosystem to bone due to its selective bone affinity. The conjugated nanosystem was characterized for particle size by TEM (average 36 nm) and morphology by AFM depicting surface irregularities due to ZOL conjugation on the surface of nanoparticles. NMR spectral data also showed the involvement of terminal -OH group of PLGA in bond formation with ZOL. Bone localization studies showed higher accumulation of the ZOL-conjugated nanosystem in bone than non-conjugated nanoparticles. This was confirmed with bone mineral affinity and specificity assay wherein the conjugated nanosystem was found to selectively bind to hydroxyapatite in comparison to other bone minerals. The biodistribution studies depicted that the conjugated nanosystem was selectively targeted to the bone area with concentrations of methotrexate reaching up to 127.4 ± 1.41 μg in 1 h. Hence, this multipronged approach using (1) ultrasmall size of nanoparticles, (2) bone selective polymer and (3) suitable bone-targeting agent resulted in mutual synergism for the specific delivery of the anti-tumour agent to the bone.

Development of novel bisphosphonate prodrugs of doxorubicin for targeting bone metastases that are cleaved pH dependently or by cathepsin B: synthesis, cleavage properties, and binding properties to hydroxyapatite as well as bone matrix

Bone metastases are a frequent cause of morbidity in cancer patients. The present palliative therapeutic options are chemotherapy, hormone therapy, and the administration of bisphosphonates. The affinity between bisphosphonates and the apatite structure of bone metastases is strong. Thus, we designed two low-molecular-weight and water-soluble prodrugs which incorporate a bisphosphonate group as a bone targeting ligand, doxorubicin as the anticancer agent, and either an acid-sensitive bond (1) or a cathepsin B cleavable bond (3) for ensuring an effective release of doxorubicin at the site of action. Cleavage studies of both prodrugs showed a fast release of doxorubicin but sufficient stability over several hours in human plasma. Effective binding of prodrug 1 and 3 was demonstrated with hydroxyapatite and with native bone. In orientating toxicity studies in nude mice, the MTD of 1 was 3-fold higher compared to conventional doxorubicin, whereas 3 showed essentially the same MTD as doxorubicin.

Bisphosphonates as radionuclide carriers for imaging or systemic therapy

Bisphosphonates (BP's), biologically stable analogs of naturally occurring pyrophosphates, became the treatment of choice for pathologic conditions characterized by increased osteoclast-mediated bone resorption, namely Paget's disease, osteoporosis and tumor bone disease. Moreover, the clinical success of BP's is also associated with their use in (99m)Tc-based radiopharmaceuticals for bone imaging. In addition to the successful delivery of (99m)Tc (γ-emitter) to bone, BP's have also been used to deliver β(-)-particle emitting radiometals (e.g.(153)Sm, (186/188)Re) for bone-pain palliation. The main goal of this Review is to update the most recent research efforts toward the synthesis, characterization and biological evaluation of novel BP-containing radiometal complexes and radiohalogenated compounds for diagnostic or therapeutic purposes. The structure and in vivo properties of those compounds will be discussed and compared to the clinically available ones, namely in terms of image quality and therapeutic effect. We will also mention briefly the use of BP's as carriers of multimodal nuclear and optical imaging probes.

N⁴-[Alkyl-(hydroxyphosphono)phosphonate]-cytidine-new drugs covalently linking antimetabolites (5-FdU, araU or AZT) with bone-targeting bisphosphonates (alendronate or pamidronate)

Amino-bisphosphonates (alendronate, pamidronate) were covalently linked in a three step synthesis, with protected and triazolylated derivatives of therapeutically used nucleoside analogs (5-FdU, araC, AZT) by substitution of their triazolyl residue. From the deprotected and chromatographically purified reaction mixtures N⁴-[alkyl-(hydroxyphosphono) phosphonate]-cytidine combining two differently cytotoxic functions were obtained. This new family of bisphosphonates (BPs) contains as novelty an alkyl side chain with a cytotoxic nucleoside. The BPs moiety allows for a high binding to hydroxyapatite which is a prerequisite for bone targeting of the drugs. In vitro binding of 5-FdU-alendronate (5-FdU-ale) to hydroxyapatite showed a sixfold increased binding of these BPs as compared to 5-FdU. Exploratory cytotoxic properties of 5-FdU-ale were tested on a panel of human tumor cell lines resulting in growth inhibition ranging between 5% and 38%. The determination of IC₅₀-concentrations of the conjugate in Lewis lung carcinoma and murine macrophages showed an incubation time dependent growth inhibition with higher sensitivity towards the tumor cells. We assume that the antimetabolite-BPs can be cleaved into different active metabolites that may exert cytotoxic and other therapeutic effects. However, the underlying mechanisms of these promising new antimetabolite-BPs conjugates remain to be evaluated in future experiments.

A promising approach for treatment of tumor-induced bone diseases: utilizing bisphosphonate derivatives of nucleoside antimetabolites

Despite palliative treatments, tumor-induced bone disease (TIBD) remains highly debilitating for many cancer patients and progression typically results in death within two years. Therefore, more effective therapies with enhanced anti-resorptive and cytotoxic characteristics are needed. We developed bisphosphonate-chemotherapeutic conjugates designed to bind bone and hydrolyze, releasing both compounds, thereby targeting both osteoclasts and tumor cells. This study examined the effects of our lead compound, MBC-11 (the anhydride formed between arabinocytidine (AraC)-5'-phosphate and etidronate), on bone tumor burden, bone volume, femur bone mineral density (BMD), and overall survival using two distinct mouse models of TIBD, the 4T1/luc breast cancer and the KAS-6/1-MIP1alpha multiple myeloma models. In mice orthotopically inoculated with 4T1/luc mouse mammary cells, MBC-11 (0.04 microg/day; s.c.) reduced the incidence of bone metastases to 40% (4/10), compared to 90% (9/10; p=0.057) and 100% (5/5; p=0.04) of PBS- or similarly-dosed, zoledronate-treated mice, respectively. MBC-11 also significantly decreased bone tumor burden compared to PBS- or zoledronate-treated mice (p=0.021, p=0.017, respectively). MBC-11 and zoledronate (0.04 microg/day) significantly increased bone volume by two- and four-fold, respectively, compared to PBS-treated mice (p=0.005, p<0.001, respectively). In mice systemically injected with human multiple myeloma KAS-6/1-MIP1alpha cells, 0.04 and 4.0 microg/day MBC-11 improved femur BMD by 13% and 16%, respectively, compared to PBS (p=0.025, p=0.017, respectively) at 10 weeks post-tumor cell injection and increased mean survival to 95 days compared to 77 days in mice treated with PBS (p=0.047). Similar doses of zoledronate also improved femur BMD (p< or =0.01 vs PBS) and increased mean survival to 86 days, but this was not significantly different than in PBS-treated mice (p=0.53). These results demonstrate that MBC-11 decreases bone tumor burden, maintains bone structure, and may increase overall survival, warranting further investigation as a treatment for TIBD.

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.

Growth factor delivery for bone tissue engineering

Bone is a dynamic tissue that undergoes significant turnover during the life cycle of an individual. Despite having a significant regenerative capability, trauma and other pathological scenarios commonly require therapeutic intervention to facilitate the healing process. Bone tissue engineering, where cellular and biological processes at a site are deliberately manipulated for a therapeutic outcome, offers a viable option for the treatment of skeletal diseases. In this review paper, we aim to provide a brief synopsis of cellular and molecular basis of bone formation that are pertinent to current efforts of bone healing. Different approaches for engineering bone tissue were presented with special emphasis on the use of soluble (diffusible) therapeutic agents to accelerate bone healing. The latter agents have been used for both local bone repair (i.e. introduction of agents directly to a site of repair) as well as systemic bone regeneration (i.e. delivery for regeneration throughout the skeletal system). Critical drug delivery and targeting issues pertinent for each mode of bone regeneration are provided. In addition, future challenges and opportunities in bone tissue engineering are proposed from the authors' perspective.

Prostaglandin E2-bisphosphonate conjugates: potential agents for treatment of osteoporosis

Conjugates of bisphosphonates (potential bone resorption inhibitors) and prostaglandin E2 (a bone formation enhancer) were prepared and evaluated for their ability to bind to bone and to liberate, enzymatically, free PGE2. The conjugate 3, an amide at C-1 of PGE2 proved to be too stable in vivo while conjugate 6, a thioester, was too labile. Several PGE2, C-15 ester-linked conjugates (18, 23, 24 and 31) were prepared and conjugate 23 was found to bind effectively to bone in vitro and in vivo and to liberate PGE2 at an acceptable rate. A 4-week study in a rat model of osteoporosis showed that 23 was better tolerated and more effective as a bone growth stimulant than daily maximum tolerated doses of free PGE2.