Bisphosphonate Derivatives of Nucleoside Antimetabolites: Hydrolytic Stability and Hydroxyapatite Adsorption of 5′-β,γ-Methylene and 5′-β,γ-(1-Hydroxyethylidene) Triphosphates of 5-Fluorouridine and ara-Cytidine

Engineering small-molecule and protein drugs for targeting bone tumors

Bone cancer is common and severe. Both primary (e.g., osteosarcoma, Ewing sarcoma) and secondary (e.g., metastatic) bone cancers lead to significant health problems and death. Currently, treatments such as chemotherapy, hormone therapy, and radiation therapy are used to treat bone cancer, but they often only shrink or slow tumor growth and do not eliminate cancer completely. The bone microenvironment contributes unique signals that influence cancer growth, immunogenicity, and metastasis. Traditional cancer therapies have limited effectiveness due to off-target effects and poor distribution on bones. As a result, therapies with improved specificity and efficacy for treating bone tumors are highly needed. One of the most promising strategies involves the targeted delivery of pharmaceutical agents to the site of bone cancer by introduction of bone-targeting moieties, such as bisphosphonates or oligopeptides. These moieties have high affinities to the bone hydroxyapatite matrix, a structure found exclusively in skeletal tissue, and can enhance the targeting ability and efficacy of anticancer drugs when combating bone tumors. This review focuses on the engineering of small molecules and proteins with bone-targeting moieties for the treatment of bone tumors.

Gemcitabine-Ibandronate Conjugate Enables the Bone-Targeted Combination Therapy in Bone Cancer: Synthesis and Efficacy in Combination with Docetaxel

Patients with cancer-induced bone disease, including primary bone cancers such as osteosarcoma (OS) and metastases from other tissues of origin, present a high unmet medical need. We present a potential therapeutic approach built upon a proven bone-targeting bisphosphonate conjugate platform with the known synergies of gemcitabine (GEM) and docetaxel (DTX). The synthesis of rationally designed GEM-IB, the conjugate of GEM-5'-phosphate with ibandronate (IB), is presented. GEM-IB as a single agent or in combination with DTX demonstrated reduced tumor burden, preservation of the bone architecture, and improved the survival in a murine model of OS. This is the first demonstration of a bone-targeting conjugate in combination with a second drug to create effective drug ratios in the bone compartment.

"The use of bisphosphonates to treat skeletal complications in solid tumours"

The skeleton is the most common site of secondary disease in breast cancer and prostate cancer, with up to 80% of patients with advanced disease developing bone metastases (BM). The proportion is also substantial in advanced lung cancer (20%-40%). Because of the high prevalence of cancers of the breast, prostate and lung, these cancers account for more than 80% of cases of metastatic bone disease occurring in solid tumours. Metastatic bone disease is associated with greatly increased bone resorption by osteoclasts, leading to moderate to severe pain and other skeletal complications, with major impact on quality of life (QoL). Skeletal Related Events (SREs) have been defined as: pathological long bone or vertebral fractures; spinal cord compression; need for radiation for pain relief or to prevent fracture/spinal cord compression, need for surgery to bone and hypercalcaemia. More recently, Symptomatic Skeletal Events (SSEs) have been defined to monitor QoL. Although there are currently no curative treatments for metastatic bone disease, patients with breast or prostate cancer and BM are now surviving for several years and sometimes longer, and prevention of SREs is the key aim to optimization of QoL. Since their discovery 50 years ago and their introduction more than 30 years ago into the field of metastatic bone disease, a range of oral and intravenous bisphosphonate drugs have made a major contribution to prevention of SREs. Large trials have clearly demonstrated the clinical value of different bisphosphonate-based drugs (including the oral drugs ibandronate and clodronate and intravenous agents such as zoledronate and pamidronate), in treatment of hypercalcaemia of malignancy and the reduction of SREs and SSEs in a range of cancers. Despite the success of denosumab in reducing osteolysis, bisphosphonates also remain mainstay drugs for treatment of metastatic bone disease. Recognizing the 50th Anniversary of the discovery of bisphosphonates, this review focuses on their continuing value in BM treatment and their future potential, for example in providing a bone-targeting vehicle for cytotoxic drugs.

First-in-Human Phase I Study of MBC-11, a Novel Bone-Targeted Cytarabine-Etidronate Conjugate in Patients with Cancer-Induced Bone Disease

Lessons Learned.

Results are consistent with MBC‐11 targeting and treating cancer‐induced bone lesions by concentrating cytarabine and etidronate at the site of disease.

MBC‐11 was well tolerated, with an maximum tolerated dose of 5 mg/kg per day and myelosuppression as the principal toxicity.

Treatment significantly reduced cancer cell activity in over half of bone lesions detected at baseline.

MBC‐11 pharmacokinetic and pharmacodynamic parameters are consistent with the novel drug design goals, and encouraging results warrant further clinical development.

Background.

MBC‐11 is a first‐in‐class conjugate of the bone‐targeting bisphosphonate etidronate covalently linked to the antimetabolite cytarabine (araC). This first‐in‐human phase I dose escalation study assessed safety, tolerability, maximum tolerated dose (MTD), plasma pharmacokinetics, bone turnover, tumor biomarkers, and bone lesion activity by fluorodeoxyglucose positron emission tomography/computed tomography (¹⁸F‐FDG‐PET/CT) imaging.

Methods.

Fifteen patients with advanced solid cancers and cancer‐induced bone disease (CIBD) were treated with 0.5–10 mg/kg per day of MBC‐11 administered daily for 5 days of every 4 weeks for up to four cycles.

Results.

Grade 1–2 myelosuppression, involving all lineages, was the principal toxicity. Two of three patients treated with 10 mg/kg experienced dose‐limiting grade 4 neutropenia and thrombocytopenia (adverse event [AE] duration ≤5 days); the MTD was 5 mg/kg. Four of five patients with pretreatment elevations of the bone resorption marker TRAP5b (tartrate resistant acid phosphatase‐5b) had persistent decrements. Six of 13 patients who reported baseline pain noted a reduction after MBC‐11. ¹⁸F‐FDG‐PET/CT imaging demonstrated partial metabolic responses in three patients and stable metabolic responses in three other patients. SUV_max (standard unit of emission normalized to total uptake) was reduced by at least 25% in 110 (52%) of 211 bone lesions. Significant activity was noted across all doses, and myelosuppression increased with dose.

Conclusion.

At MBC‐11 doses that were well tolerated, substantial reductions in metabolic activity of bone‐associated cancer cells provide a foundation for further disease‐directed efficacy studies.

Bisphosphonate conjugation for bone specific drug targeting

Bones provide essential functions and are sites of unique biochemistry and specialized cells, but can also be sites of disease. The treatment of bone disorders and neoplasia has presented difficulties in the past, and improved delivery of drugs to bone remains an important goal for achieving effective treatments. Drug targeting strategies have improved drug localization to bone by taking advantage of the high mineral concentration unique to the bone hydroxyapatite matrix, as well as tissue-specific cell types. The bisphosphonate molecule class binds specifically to hydroxyapatite and inhibits osteoclast resorption of bone, providing direct treatment for degenerative bone disorders, and as emerging evidence suggests, cancer. These bone-binding molecules also provide the opportunity to deliver other drugs specifically to bone by bisphosphonate conjugation. Bisphosphonate bone-targeted therapies have been successful in treatment of osteoporosis, primary and metastatic neoplasms of the bone, and other bone disorders, as well as refining bone imaging. In this review, we focus upon the use of bisphosphonate conjugates with antineoplastic agents, and overview bisphosphonate based imaging agents, nanoparticles, and other drugs. We also discuss linker design potential and the current state of bisphosphonate conjugate research progress. Ongoing investigations continue to expand the possibilities for bone-targeted therapeutics and for extending their reach into clinical practice.

Metalated nucleotide chemisorption on hydroxyapatite

The experiments here reported evidence on the importance of the residual charge of a nucleotide derivative, for the adsorption on nHAP (hydroxyapatite nanocrystals), in water solution. We found that the simple presence of phosphates on the nucleotide derivative does not guarantee adsorption on nHAP. On the other hand, we demonstrated that a cationic or neutral charge on a nucleotide derivative produces a strongly reduced chemical adsorption (chemisorption) whereas, in the presence of a net negative charge, relevant adsorption on nHAP is observed. The number of phosphates can only modulate the adsorption efficiency of a molecule provided that this latter bears an overall negative charge. The neutral zwitterionic nucleotide Pt(II) complexes, bearing negatively charged phosphates, are unable to give stable chemisorption. Previous considerations are important to model the binding ability of phosphate bearing nucleotide derivatives or molecules on hydroxyapatite. The findings reported in the present paper could be relevant in bone tissue targeting or nHAP mediated drug delivery.

Phosphate analogues in the dissection of mechanism

Phosphoryl group transfer is central to genetic replication, cellular signalling and many metabolic processes. Understanding the mechanisms of phosphorylation and phosphate ester and anhydride cleavage is key to efforts towards biotechnological and biomedical exploitation of phosphate-handling enzymes. Analogues of phosphate esters and anhydrides are indispensable tools, alongside protein mutagenesis and computational methods, for the dissection of phosphoryl transfer mechanisms. Hydrolysable and non-hydrolysable phosphate analogues have provided insight into the nature and sites of phosphoryl transfer processes. Kinetic isotope effects and crystallography using transition state analogues have painted more detailed pictures of transition states and how enzymes work to stabilise them.

Enhanced affinity bifunctional bisphosphonates for targeted delivery of therapeutic agents to bone

Skeletal diseases have a major impact on the worldwide population and economy. Although several therapeutic agents and treatments are available for addressing bone diseases, they are not being fully utilized because of their uptake in nontargeted sites and related side effects. Active targeting with controlled delivery is an ideal approach for treatment of such diseases. Because bisphosphonates are known to have high affinity to bone and are being widely used in treatment of osteoporosis, they are well-suited for drug targeting to bone. In this study, a targeted delivery of therapeutic agent to resorption sites and wound healing sites of bone was explored. Toward this goal, bifunctional hydrazine-bisphosphonates (HBPs), with spacers of various lengths, were synthesized and studied for their enhanced affinity to bone. Crystal growth inhibition studies showed that these HBPs have high affinity to hydroxyapatite, and HBPs with shorter spacers bind more strongly than alendronate to hydroxyapatite. The HBPs did not affect proliferation of MC3T3-E1 preosteoblasts, did not induce apoptosis, and were not cytotoxic at the concentration range tested (10(-6)-10(-4) M). Furthermore, drugs can be linked to the HBPs through a hydrazone linkage that is cleavable at the low pH of bone resorption and wound healing sites, leading to release of the drug. This was demonstrated using hydroxyapatite as a model material of bone and 4-nitrobenzaldehyde as a model drug. This study suggests that these HBPs could be used for targeted delivery of therapeutic agents to bone.

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.

Modifications to the dNTP triphosphate moiety: from mechanistic probes for DNA polymerases to antiviral and anti-cancer drug design

Abnormal replication of DNA is associated with many important human diseases, most notably viral infections and neoplasms. Existing approaches to chemotherapeutics for diseases associated with dysfunctional DNA replication classically involve nucleoside analogues that inhibit polymerase activity due to modification in the nucleobase and/or ribose moieties. These compounds must undergo multiple phosphorylation steps in vivo, converting them into triphosphosphates, in order to inhibit their targeted DNA polymerase. Nucleotide monophosphonates enable bypassing the initial phosphorylation step at the cost of decreased bioavailability. Relatively little attention has been paid to higher nucleotides (corresponding to the natural di- and triphosphate DNA polymerase substrates) as drug platforms due to their expected poor deliverability. However, a better understanding of DNA polymerase mechanism and fidelity dependence on the triphosphate moiety is beginning to emerge, aided by systematic incorporation into this group of substituted methylenebisphosphonate probes. Meanwhile, other bridging, as well as non-bridging, modifications have revealed intriguing possibilities for new drug design. We briefly survey some of this recent work, and argue that the potential of nucleotide-based drugs, and intriguing preliminary progress in this area, warrant acceptance of the challenges that they present with respect to bioavailability and metabolic stability.