There is increasing evidence that extracellular ATP acts directly on bone cells via P2 receptors. In normal rat osteoclasts, ATP activates both non‐selective cation channels and Ca2+‐dependent K+ channels. In this study we investigated the action of ATP on the formation of osteoclasts and on the ultimate function of these cells, namely resorption pit formation. We found that ATP stimulated resorption pit formation up to 5.6‐fold when osteoclast‐containing bone cell populations from neonatal rats were cultured for 26 h on ivory discs, with a maximum effect occurring at relatively low concentrations (0.2‐2 μM). The stimulatory effect of ATP was amplified greatly when osteoclasts were activated by culture in acidified media (pH 6.9‐7.0). Pit formation by acid‐activated osteoclasts in the absence of ATP was inhibited by apyrase, an ecto‐ATPase and by suramin, an antagonist of P2 receptors. Over the same concentration range at which rat osteoclast activation occurred (0.2‐2 μM), ATP also enhanced osteoclast formation in 10 day mouse marrow cultures, by up to 3.3‐fold, with corresponding increases in resorption pit formation. Higher concentrations of ATP (20–200 μM) reduced or blocked osteoclast formation. Adenosine, a P1 receptor agonist, was without effect on either osteoclast activation or formation. These results suggest that low levels of extracellular ATP may play a fundamental role in modulating both the resorptive function and formation of mammalian osteoclasts.

Arginine-glycine-aspartate (RGD)-binding a V b 3 -integrin and a V b 5 -integrin play key roles in tumor angiogenesis. We examined an 18 F-labeled small peptide (fluciclatide [United States Adopted Name (ASAN)-approved, International Nonproprietary Name (INN)-proposed name], previously referred to as AH111585) containing an RGD sequence. Fluciclatide binds with a high (nM) affinity to a V b 3 -integrin and a V b 5 -integrin, which are highly expressed on tumors and the tumor neovasculature. In this study, 18 F-fluciclatide was used to examine the response of human glioblastoma xenografts to treatment with the antiangiogenic agent sunitinib. Methods: U87-MG tumor uptake of 18 F-fluciclatide was determined by small-animal PET after longitudinal administration of the antiangiogenic agent sunitinib (a 2-wk dosing regimen). Tumor sizes were measured throughout the study, and tumor volumes were calculated. Tumor microvessel density (MVD) after therapy was also analyzed. Results: Dynamic small-animal PET of 18 F-fluciclatide uptake after administration of the clinically relevant antiangiogenic agent sunitinib revealed a reduction in the tumor uptake of 18 F-fluciclatide compared with that in vehicle-treated controls over the 2-wk dosing regimen. Skeletal muscle, used as a reference tissue, showed equivalent 18 F-fluciclatide uptake in both therapy and control groups. A reduction in tumor MVD was also observed after treatment with the antiangiogenic agent. No significant changes in tumor volume were observed in the 2 groups. Conclusion: The data demonstrated that 18 F-fluciclatide detected changes in tumor uptake after acute antiangiogenic therapy markedly earlier than any significant volumetric changes were observable. These results suggest that this imaging agent may provide clinically important information for guiding patient care and monitoring the response to antiangiogenic therapy. Int egrins are a family of cell adhesion molecules consisting of 2 noncovalently bound transmembrane subunits, a and b, that form heterodimers with distinct adhesive capabilities (1). In mammals, 18 a and 8 b subunits assemble into 24 different receptors. Integrins play important roles in several pathologic processes, such as inflammation, fibrosis, tumor metastasis, and angiogenesis (2,3).Angiogenesis, the process of forming new blood vessels from existing vessels (4), is central to normal biologic processes, such as embryogenesis, tissue remodeling, inflammation, and wound healing, and is present in numerous disease states, including rheumatoid arthritis, psoriasis, restenosis, diabetic retinopathy, and tumor growth (5-7). The interest in angiogenesis research has been fueled by the potential to develop antiangiogenic drugs as novel therapeutic agents for targeting tumors and several nononcologic diseases. a V b 3 -integrin and a V b 5 -integrin act as receptors for a variety of proteins expressing the exposed arginine-glycineaspartate (RGD) tripeptide sequence, such as vitronectin, fibronectin, fibrinogen, laminin, collagen, Von Will...

Achieving high-yielding, robust, and reproducible chemistry is a prerequisite for the (18)F-labeling of peptides for quantitative receptor imaging using positron emission tomography (PET). In this study, we extend the toolbox of oxime chemistry to include the novel prosthetic groups [(18)F]-(2-{2-[2-(2-fluoroethoxy)ethoxy]ethoxy}ethoxy)acetaldehyde, [(18)F]5, and [(18)F]-4-(3-fluoropropoxy)benzaldehyde, [(18)F]9, in addition to the widely used 4-[(18)F]fluorobenzaldehyde, [(18)F]12. The three (18)F-aldehydes were conjugated to the same aminooxy-bearing RGD peptide and the effect of the prosthetic group on biodistribution and tumor uptake studied in mice. The peptide conjugate [(18)F]7 was found to possess superior in vivo pharmacokinetics with higher tumor to blood, tumor to liver, tumor to muscle, and tumor to lung ratios than either [(18)F]10 or [(18)F]13. The radioactivity from the [(18)F]7 conjugate excreted more extensively through the kidney route with 79%id passing through the urine and bladder at the 2 h time point compared to around 55%id for the more hydrophobic conjugates [(18)F]10 and [(18)F]13. The chemical nature of a prosthetic group can be employed to tailor the overall biodistribution profile of the radiotracer. In this example, the hydrophilic nature of the ethylene glycol containing prosthetic group [(18)F]5 clearly influences the overall excretion pattern for the RGD peptide conjugate.

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We examined the effects of HCO(3)(-) and CO(2) acidosis on osteoclast-mediated Ca(2+) release from 3-day cultures of neonatal mouse calvaria. Ca(2+) release was minimal above pH 7.2 in control cultures but was stimulated strongly by the addition of small amounts of H(+) to culture medium (HCO(3)(-) acidosis). For example, addition of 4 meq/l H(+) reduced pH from 7.12 to 7.03 and increased Ca(2+) release 3.8-fold. The largest stimulatory effects (8- to 11-fold), observed with 15-16 meq/l added H(+), were comparable to the maximal Ca(2+) release elicited by 1,25-dihydroxyvitamin D(3) [1, 25(OH)(2)D(3); 10 nM], parathyroid hormone (10 nM), or prostaglandin E(2) (1 microM); the action of these osteolytic agents was attenuated strongly when ambient pH was increased from approximately 7.1 to approximately 7.3. CO(2) acidosis was a less effective stimulator of Ca(2+) release than HCO(3)(-) acidosis over a similar pH range. Ca(2+) release stimulated by HCO(3)(-) acidosis was almost completely blocked by salmon calcitonin (20 ng/ml), implying osteoclast involvement. In whole mount preparations of control half-calvaria, approximately 400 inactive osteoclast-like multinucleate cells were present; in calvaria exposed to HCO(3)(-) acidosis and to the other osteolytic agents studied, extensive osteoclastic resorption, with perforation of bones, was visible. HCO(3)(-) acidosis, however, reduced numbers of osteoclast-like cells by approximately 50%, whereas 1,25(OH)(2)D(3) treatment caused increases of approximately 75%. The results suggest that HCO(3)(-) acidosis stimulates resorption by activating mature osteoclasts already present in calvarial bones, rather than by inducing formation of new osteoclasts, and provide further support for the critical role of acid-base balance in controlling osteoclast function.