Relative binding affinities of bisphosphonates for human bone and relationship to antiresorptive efficacy
Introduction
The bisphosphonates (BPs) are the most effective antiresorptive agents used for the clinical treatment of postmenopausal and other forms of osteoporosis. Several factors contribute to their potency and efficacy as inhibitors of bone resorption, the intracellular target of BPs being one major component. To this extent, the relative IC50s of the nitrogen-containing BPs (N-BPs) such as alendronate (ALN) and risedronate (RIS) for inhibiting their intracellular target, farnesyl diphosphate synthase, fall into the low to mid nanomolar range [4], [7], [40]. For these and other N-BPs, a correlation exists between in vitro potency to inhibit this enzyme and the lowest effective dose in the rat [7]. Clodronate (CL2) falls into a different category regarding intracellular mechanism [13]. For this BP, and probably etidronate (EHDP) and tiludronate (TIL), the BP itself is incorporated into the β and γ positions of ATP through an interaction with aminoacyl-tRNA synthetases. The resulting analog is toxic and causes osteoclast apoptosis. The different mechanisms make difficult the process of comparing intracellular potency between different classes of BP. One can nonetheless make comparisons based upon the relative potency to induce cellular responses such as suppression of bone resorption and induction of osteoclast apoptosis in vitro. In most regards, CL2 is considered to be about one-third as potent as ALN and RIS in these types of in vitro assays.
Osteoclast apoptosis is one cellular mechanism by which osteoclastic bone resorption can be inhibited by a BP. Whereas the non-nitrogen-containing BPs such as CL2 and EHDP employ apoptosis as the dominant mechanism for antiresorptive efficacy, the N-BPs employ this as a secondary mechanism [1], [14]. Nonetheless, since all antiresorptive BPs induce osteoclast apoptosis, this endpoint can serve as an effective in vitro measure of relative potency. In several studies of BP-induced cultured osteoclast apoptosis, ALN, RIS and CL2 all induced cell death within a relatively narrow concentration range of three-fold [3], [8], [14], [17], [19], [30]. Similarly, we previously found that CL2 should be dosed at 1.7-fold higher than ALN or RIS in head-to-head in vitro bone resorption assays in order to achieve a similar antiresorptive effect [14]. In these assays, the BPs were allowed to bathe the bone during the entire assay, and no attempt was made to wash away the unattached BP prior to addition of osteoclast cultures. Based on these in vitro observations, one might predict that in vivo or clinical potency for these three BPs could fall into the same type of narrow range.
Contrary to in vitro predictions, CL2 is substantially less potent than ALN or RIS in vivo than expected. Although ALN and CL2 have never been compared head-to-head in a clinical study, they have been compared in rats in vivo with CL2 dosing set at surprisingly higher values of up to 100-fold [9], [36]. A head to head clinical comparison showed that ALN and RIS increase lumbar spine bone mineral density by 3.7% and 2.6%, respectively, after 1 year [32]. The ability of these BPs to elicit these changes in BMD at relatively low daily (or weekly) doses is striking when compared to the substantially weaker responses seen in the clinic with CL2, which shows no increase in lumbar spine BMD after 3 years of dosing at 65 mg per day [37]. Increases of 3.8% can be seen after 3 years treatment with the substantially higher oral daily dose of 800 mg. This suggests a failure of standard in vitro testing to predict either in vivo animal or clinical potency of CL2 vs. ALN or RIS.
In the present study, we sought to establish how differences in relative binding affinities of clinically tested BPs for human bone could serve as independent contributors to antiresorptive efficacy. Some of the earliest studies on the subject of BP affinity examined inhibition of hydroxyapatite (HAP) mineral formation [10], [12]. Although the earlier studies did not directly compare BPs in modern clinical use, they did establish, for instance, that CL2 has substantially lower affinity for calcium phosphate [35]. Recent studies have employed similar approaches to compare relative binding affinities of BPs such as ALN, EHDP, RIS and zoledronate (ZOL) for inhibition of both hydroxyapatite and octa calcium phosphate crystal formation (or dissolution) as a surrogate for binding to human bone [26]. In an effort to make these studies more relevant to bone, which is comprised of ∼ 35% protein, inhibition of the formation of carbonated apatite crystals (CAP) has also been tested [15]. The data comparing ALN and RIS inhibition of HAP, CAP or octa calcium phosphate suggest a 1.4- to 2.6-fold greater affinity for ALN, depending on the system. The affinity constants for ALN using these chemical in vitro methods fall into the mid nanomolar to low micromolar range. Meanwhile, ALN was found to bind human bone with an apparent Kd of 1 mM [34], and competition binding studies suggest an IC50 for binding to mouse bone in the 70–100 μM range [41]. To date, only ALN and EHDP have been compared for their affinities to human bone, and the data from competition binding analyses suggest that the binding is identical [34]. Likewise, ALN and RIS show similar binding to mouse metatarsals in competition binding analyses [41]. These observations differ from those describing relative effects of ALN and EHDP on in vitro crystal growth, whereby an ALN potency advantage has been reported, depending on the type of crystal [26]. Thus, substantial differences may exist in how BPs affect crystal growth vs. how they bind to human or mouse bone. This suggests the need for a more comprehensive comparison of the various BPs.
In the present study, we measured relative binding affinities for clinically tested BPs for human bone. In addition to measuring the binding affinity and on and off rates for ALN binding, a competition binding approach was used to compare between the clinically relevant BPs. We find that CL2, with a sub mM affinity, bound ∼10-fold more weakly than most tested BPs. All BPs with an OH-group attached to the central carbon bound within a fairly narrow affinity range. This suggests that structural differences in BPs that relate to intracellular potency have little, if any, effect on binding to human bone under these conditions.
Section snippets
Materials
Human bone particles were collected by sifting through a 180 μm mesh screen and then capturing in a 150 μm mesh screen, as previously described [34]. [14C]-ALN and all non-radioactive BPs were from Merck Research Laboratories. Tris–formate, ATP, AMP, a second sample of CL2, pyrophosphate, beta-glycerophosphate and sodium phosphate were all from Sigma-Aldrich.
Binding of [14C]-ALN to human bone particles
Human bone particles were prewashed twice with binding buffer (0.2 M Tris–formate, pH 7.2) at 22°C. The choice of this buffer and
ALN binding to human bone particles
In an effort to elucidate possible differences between BPs that might influence in vivo or clinical potency, we examined the relative binding affinities of BPs towards human bone. In vivo studies of ALN localization on rat bone showed that it preferentially binds to sites of osteoclastic resorption with eight-fold higher BP found beneath the osteoclast than seen on osteoblast-covered surfaces [23]. Resorption sites represent the portions of bone that are the most exposed since osteoclasts are
Discussion
In this study, we examined the equilibrium bone binding affinities of clinically tested BPs as a putative factor influencing antiresorptive potency and efficacy. In contrast to several studies examining effects of BPs on HAP crystal growth or dissolution in vitro published decades ago [6], [10], [12], [35], we assessed binding to human bone with an alternative method. With this approach, we found that the binding affinity of the clinically tested OH-BPs fell into a narrow range. Strikingly, CL2
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Current address: Department of Biochemistry and Biophysics, University of Pennsylvania School of Medicine, Philadelphia, PA 19104, USA.