Novel vitamin B6 derivative of alendronate rapidly absorbed following oral administration

Abstract

Poor pharmacokinetics and upper GI irritation associated with oral bisphosphonate therapies are significant drawbacks to otherwise effective treatments for osteoporosis and other metabolic bone diseases. To address these problems we have attempted to provide a means for facilitated uptake of bisphosphonate via covalent attachment of vitamin B6. The chemistry of attachment is amenable to any bisphosphonate containing a free primary amino group, such as pamidronate and alendronate. We used 14C-radiolabeled alendronate and labeled alendronate-vitamin B6 conjugate MBC-31 for pharmacokinetic studies. In rats dosed orally, the Cmax of MBC31 (~6.3 μg/ml) was double that of alendronate, and the Tmax of MBC-31 was ~5 minutes as compared to ~30 minutes for alendronate. While the oral bioavailability (~0.7%) was essentially unchanged by conjugation, the clearance of MBC-31 from plasma was nearly doubled and may suggest an increase in tissue absorption. Furthermore, the distribution of alendronate and MBC31 to bone appeared to be equivalent. Recent data from an estrogen deficiency-induced bone loss ovariectomized rat model of osteoporosis showed an MBC-31 induced reduction of serum and urine biomarkers of bone turnover and an improvement in total bone density. INTRODUCTION While millions of post menopausal women are currently taking bisphosphonates for the prevention and treatment of osteoporosis, less than half adhere to therapy regimens [1, 2]. Non-compliance is largely attributed to the following dosing requirements, i. no prior food, ii. the pill is taken with water, iii. no food or drink for at least 30 minutes post-dosing and iv. the person must remain in an upright position for at least 30 minutes. We report our attempt to overcome these problems by improving the pharmacokinetic properties of alendronate via conjugating with vitamin B6. It has been shown, that vitamin B6 absorption from the food undergoes a specific transporter-facilitated mechanism [3, 4, 5]. FIGURE 1. Intracellular Processing of Pyridoxylamines [6] Furthermore, Zhang and McCormick [6] (Fig.1) have shown that many amines bearing a pyridoxyl group on the nitrogen enter cells by a transport system for the various forms of vitamin B6 and, once inside, they are phosphorylated on O-5' by pyridoxal kinase (EC 2.7.1.35) and then oxidized by pyridoxaminephosphate oxidase (EC 1.4.3.5) to release pyrdoxal phosphate and the amine. Among the amines shown to behave in this way was the amino acid β-alanine. This leads us to believe that there is a high likelihood that metabolic degradation of B6-conjugates will result in the release of free bisphosphonate and pyridoxal 5’-phosphate. The broader applicability of N-pyridoxylamines for delivery of amines can be appreciated when it is realized that almost all facultative and aerobic cells have both pyridoxal kinase and pyridoxamine-phosphate oxidase [5]. However, it is also possible that the intact compound will inhibit the same target enzyme (FPPS, Farnesyl diphosphate synthase, EC 2.5.1.10) of aminobisphosphonates demonstrating antiresorptive and/or anti-cancer activity. = + _