Current Pharmaceutical Design - Volume 8, Issue 21, 2002

Postmenopausal women are at greatest risk of rapid bone loss and fracture with glucocorticoids and should be actively considered for prophylactic measures. In men and premenopausal women receiving glucocorticoids, the decision to use anti-osteoporosis prophylaxis is less clear and depends upon baseline bone mineral density [BMD], anticipated dose and duration of glucocorticoids. Based upon evidence the order of choice for prophylaxis would be a bisphosphonate followed by a vitamin D metabolite or hormone replacement therapy [HRT]. Calcium alone appears unable to prevent rapid bone loss in patients starting glucocorticoids. HRT should clearly be considered if hypogonadism is present. In patients receiving chronic low dose glucocorticoids, treatment with calcium and vitamin D may be sufficient to prevent further bone loss. However since fracture risk is a function of multiple factors including the degree of reduction in BMD as well as the duration of exposure, treatment with therapy to increase BMD will reduce fracture risk even in patients receiving chronic low dose glucocorticoids.

During the past ten years the range of treatments available for patients with osteoporosis has increased greatly. A decade ago the only proven therapy was oestrogen, while today the choice includes bisphosphonates, selective oestrogen receptor modulators, calcitonin, calcium and vitamin D supplementation and, in the near future, parathyroid hormone. Clinical trials involving bone mineral density (BMD) scans of the spine and femur have had an important role in the evaluation of these new therapies. Supplementary information about treatments has been provided by BMD scans of the total body and distal radius as well as by measurements of biochemical markers of bone turnover in serum and urine. Most important of all, the efficacy of treatments has been verified in large trials powered to show reductions in fracture risk. In routine clinical use, BMD scanning has an important role in identifying individual patients with osteoporosis and helping to make decisions about their treatment. However, in contrast to the use of BMD scans in clinical trials, their value for monitoring response to therapy in individual patients is less certain because in many cases the increases in BMD are too small to reliably distinguish between true changes and measurement error. However, experience with well established therapies such as oestrogen and bisphosphonates suggests that these treatments have a beneficial effect on bone in the large majority of patients and individual monitoring of BMD is probably not necessary.

Strontium ranelate is composed of an organic moiety (ranelic acid) and of two atoms of stable nonradioactive strontium. In vitro, strontium ranelate increases collagen and non-collagenic proteins synthesis by mature osteoblast enriched cells. The effects of strontium ranelate on bone formation were confirmed as strontium ranelate enhanced pre-osteoblastic cells replication. The stimulation by strontium ranelate of the replication of osteoprogenitor cells and collagen as well as non-collagenic protein synthesis in osteoblasts provides substantial evidence to categorise SR ranelate as a bone forming agent. In the mouse calvaria culture system, SR ranelate induces a dose-dependent inhibition of labelled calcium release. The inhibitory effects of SR ranelate on bone resorption were close to those of salmon calcitonin. In the isolated rat osteoclast assay, a pre-incubation of bone slices with SR ranelate induced a dose-dependent inhibition of the bone resorbing activity of a treated rat osteoclast. SR ranelate also dose-dependently inhibited, in a chicken bone marrow culture, the expression of both CA II and the alpha-subunit of the vitronectin receptor. These effects showing that SR ranelate significantly affects bone resorption due to direct and / or matrix-mediated inhibition of osteoclast activity and also inhibits osteoclasts differenciation are compatible with the profile of an antiresorptive drug. In normal rats, administration of SR ranelate induces an improvement in the mechanical properties of the humerus and / or the lumbar vertebra associated with a commensurate increase in bone dimension, shaft and volume. This was not related to any change in the stiffness, suggesting the absence of a mineralisation defect. After oral administration of SR ranelate in humans, the absolute bio-availability of SR ranelate is 27 % after a dose of 2g is given as sachets. The simultaneous intake of SR ranelate and calcium remarkably reduces the bio-availability of SR. SR ranelate was administered in 160 early postmenopausal women, in a 24-month, double-blind, placebo-controlled, prospective randomized study. Daily oral dose of 125 mg, 500 mg, 1 g of SR ranelate were compared to a placebo. At the conclusion of the study, the percent variation of lumbar adjusted BMD from baseline was significantly different in the group receiving 1 g / day of SR as compared to placebo (+ 1.41 % versus - 0.98 % respectively). Increase in total hip and neck BMD averages respectively 3.2 % and 2.5 %. SR ranelate does not induce any significant adverse reaction compared to those observed in women receiving a placebo for the same duration. In a phase II study, the effect of SR ranelate in postmenopausal women with vertebral osteoporotic fractures were assessed during a double-blind, placebo-controlled trial. SR ranelate (500 mg, 1 g, 2 g per day) or placebo were given to 353 Caucasian women with prevalent osteoporosis. At the conclusion of this two-year study, the annual increase in lumbar adjusted BMD of the group receiving 2 g of SR ranelate was + 2.97 %. This result was significantly different as compared to placebo. A significant decrease in pyridinium crosslinks (NTX) and an increase in bone specific alkaline phosphatase were evident after 3 and 6 months of treatment. During the second year of treatment, the dose of 2 g was associated with a 4 % reduction in the number of patient experiencing a new vertebral deformity. Bone histomorphometry showed no mineralisation defects. The same percentage of withdrawal following an adverse effect was observed for patients receiving placebo and for those receiving 2 g of strontium ranelate. Currently, strontium ranelate is further investigated in a large Phase III program that includes two extensive trials for the treatment of severe osteoporosis, one assessing SR ranelate effects on the risk of vertebral fractures (SOTI) and one evaluating the effects of SR ranelate on peripheral (non spinal) fractures (TROPOS). The primary analysis of the SOTI study, evaluating the effect of 2 g of strontium ranelate on vertebral fracture rates are expected to be released during the summer 2002.

Mesenchymal Stem Cells (MSCs) are adult stem cells that constitute a variety of adult tissues. MSCs maintain self-renewal ability with the ability to give rise to different mesenchymal cells, and are therefore responsible in part, for the regenerative capacity of mesenchymal tissues. MSCs throughout a variety of species were found to be able to differentiate to several mesenchymal tissues including: bone, cartilage, stroma, adipose, connective tissue, muscle and tendon. MSCs are relatively easily isolated from the bone marrow and expanded in vitro. It was found that MSCs play an important role in bone physiology and hematopiesis, and in part participate in the pathophysiology related to bone diseases, mainly osteoporosis. MSCs were widely used in experimental studies in vivo, and were shown to form mesenchymal tissues. These discovered features have made MSCs good candidates for the development of various therapeutic modalities aimed to regenerate mesenchymal tissues, mainly bone. The more important approaches currently utilizing MSCs are gene therapy and tissue engineering. Both exploit the current knowledge in molecular biology and biomaterial science in order to direct MSCs to differentiate in vivo to desired lineages and tissues. Better understanding of the molecular mechanism directing the differentiation of MSCs, will eventually allow us to properly manipulate MSCs both in vivo and ex vivo to allow the regeneration of complex tissues and organs.

Bisphosphonates are organic analogues of pyrophosphate that are resistant to hydrolysis. Bisphosphonates exhibit an exceptional affinity to bone, which led to exploration of their utility for targeting pharmacological agents to bone. Among the pharmacological agents explored for bone delivery are radioisotopes, anti-neoplastic drugs, agents intended for augmentation of systemic bone mass, antiinflammatory drugs and proteins. This review is intended to provide a summary of the literature on bisphosphonate-based drug delivery. A survey of therapeutic agents designed for bone targeting was provided in the first part of this review. Special emphasis was placed on the preclinical performance of the developed agents. In the second part, several aspects that were considered critical for the success of bisphosphonate-based drug delivery were explored. These aspects were oral bioavailability, protein binding of BPs, the nature of a BP-drug linkage and the choice of bone-affinity molecule. This article concludes with the author's perspective on the future of bisphosphonate-based drug delivery.