Vascular calcification: not so crystal clear

[68Ga]Ga-THP-Pam: A Bisphosphonate PET Tracer with Facile Radiolabeling and Broad Calcium Mineral Affinity

Calcium minerals such as hydroxyapatite (HAp) can be detected noninvasively in vivo using nuclear imaging agents such as [18F]NaF (available from cyclotrons), for positron emission tomography (PET) and 99mTc-radiolabeled bisphosphonates (BP; available from 99mTc generators for single photon emission computed tomography (SPECT) or scintigraphy). These two types of imaging agents allow detection of bone metastases (based on the presence of HAp) and vascular calcification lesions (that contain HAp and other calcium minerals). With the aim of developing a cyclotron-independent PET radiotracer for these lesions, with broad calcium mineral affinity and simple one-step radiolabeling, we developed [68Ga]Ga-THP-Pam. Radiolabeling with 68Ga is achieved using a mild single-step kit (5 min, room temperature, pH 7) to high radiochemical yield and purity (>95%). NMR studies demonstrate that Ga binds via the THP chelator, leaving the BP free to bind to its biological target. [68Ga]Ga-THP-Pam shows high stability in human serum. The calcium mineral binding of [68Ga]Ga-THP-Pam was compared in vitro to two other 68Ga-BPs which have been successfully evaluated in humans, [68Ga]Ga-NO2APBP and [68Ga]Ga-BPAMD, as well as [18F]NaF. Interestingly, we found that all 68Ga-BPs have a high affinity for a broad range of calcium minerals implicated in vascular calcification disease, while [18F]NaF is selective for HAp. Using healthy young mice as a model of metabolically active growing calcium mineral in vivo, we compared the pharmacokinetics and biodistribution of [68Ga]Ga-THP-Pam with [18F]NaF as well as [68Ga]NO2APBP. These studies revealed that [68Ga]Ga-THP-Pam has high in vivo affinity for bone tissue (high bone/muscle and bone/blood ratios) and fast blood clearance (t1/2 < 10 min) comparable to both [68Ga]NO2APBP and [18F]NaF. Overall, [68Ga]Ga-THP-Pam shows high potential for clinical translation as a cyclotron-independent calcium mineral PET radiotracer, with simple and efficient radiochemistry that can be easily implemented in any radiopharmacy.

Cyclooxygenase-2/sclerostin mediates TGF-β1-induced calcification in vascular smooth muscle cells and rats undergoing renal failure

In this study, we studied the effect and possible mechanism of TGF-β1 on vascular calcification. We found that the serum levels of TGF-β1 and cycloxygenase-2 (COX-2) were significantly increased in patients with chronic kidney disease. Phosphate up regulated TGF-β1 in vascular smooth muscle cells (VSMCs). TGF-β1 decreased the markers of VSMCs, but increased osteogenic markers and calcification in aortic segments. The phosphate-induced osteogenic markers were reduced by the TGFβR I inhibitor (LY364947), which also attenuated the potential of phosphate to reduce VSMC markers in VSMCs. Both phosphate and TGF-β1 increased the protein level of β-catenin, which was partially mitigated by LY364947. TGF-β1 decreased sclerostin, and exogenous sclerostin decreased the mineralization induced by TGF-β1. LY364947 reduced the phosphate and TGF-β1 induced COX-2. Meanwhile, the effects of TGF-β1 on osteogenic markers, β-catenin, and sclerostin, were partially reversed by the COX-2 inhibitor. Mechanistically, we found that p-Smad2/3 and p-CREB were both enriched at the promoter regions of sclerostin and β-catenin. TGF-β1 and COX-2 were significantly elevated in serum and aorta of rats undergoing renal failure. Therapeutic administration of meloxicam effectively ameliorated the renal lesion. Our results suggested that COX-2 may mediate the effect of TGF-β1 on vascular calcification through down-regulating sclerostin in VMSCs.

Pathological calcification in osteoarthritis: an outcome or a disease initiator?

In the progression of osteoarthritis, pathological calcification in the affected joint is an important feature. The role of these crystallites in the pathogenesis and progression of osteoarthritis is controversial; it remains unclear whether they act as a disease initiator or are present as a result of joint damage. Recent studies reported that the molecular mechanisms regulating physiological calcification of skeletal tissues are similar to those regulating pathological or ectopic calcification of soft tissues. Pathological calcification takes place when the equilibrium is disrupted. Calcium phosphate crystallites are identified in most affected joints and the presence of these crystallites is closely correlated with the extent of joint destruction. These observations suggest that pathological calcification is most likely to be a disease initiator instead of an outcome of osteoarthritis progression. Inhibiting pathological crystallite deposition within joint tissues therefore represents a potential therapeutic target in the management of osteoarthritis.

Physicochemical characterization of human cardiovascular deposits

Detailed crystal chemical characterization of human pathological cardiovascular deposits (PCD) was conducted applying wide set of the instrumental methods (XRD, FTIR, Raman, SEM, different chemical analyses). There was some progress achieved in the understanding of it formation mechanism. The obtained data evidence that pathological cardiovascular deposits are presented by non-stoichiometric water-bearing B-type carbonated hydroxyapatite just like other apatites of the human body. But PCD apatite is characterized by higher concentration of B-type carbonate ion (up to ~ 6 wt%) which leads to the increasing influence of the carbonate-ion on the unit cell parameters in comparison with water and other substitutes. Another difference between PCD apatite and other pathogenic apatites of the human body is the smaller variations of the unit cell parameters, caused by smaller variations of the blood chemical composition. It was shown that apatite on the surface of PCD is characterized by the more non-stoichiometric composition compared to apatite inside these deposits. It is assumed that the formation mechanisms of the PCD apatite and the bone apatite may be similar.

Calciphylaxis after parathyroidectomy

A 60-year-old African American man with end stage renal disease on hemodialysis (HD) for the past 2.5 years developed severe hyperparathyroidism. Other past medical history included atrial fibrillation, type II diabetes mellitus, hypertension, gout, pericardial effusion needing pericardial window, deep vein thrombosis, mitral insufficiency, and cardiomyopathy with implantable cardioversion device placement. His parathyroid hormone (PTH) level peaked at 4,191 pg/mL despite being on cinacalcet, sevelamer, and paricalcitol. He underwent a subtotal parathyroidectomy in January 2015, after which his PTH levels dropped to 184 pg/mL. Approximately 4 weeks later he developed extensive, painful necrotic skin lesions in both his lower extremities and buttocks, suggestive of calciphylaxis which was confirmed by tissue biopsy. The patient was treated with elaborate wound care, wound debridements, increased dialysis dose, and IV sodium thiosulfate (STS) during hemodialysis. Besides STS, he was treated with narcotics, gabapentin, topical lidocaine on intact skin, and oral steroids for pain control. Even though his lesions improved initially, he deteriorated due to recurrent sepsis, respiratory failure, and prolonged hospitalization which culminated in stopping dialysis before he passed away. Calciphylaxis, or calcific uremic arteriolopathy, is a life-threatening complication of end stage renal disease. Treatment of this condition is multidisciplinary which includes elaborate wound care, increasing dialysis dose, and discontinuing vitamin D supplements and calcium containing phosphate binders. Even though STS has been recommended off-label, several studies have shown promising results with resolution of lesions. Thus, sodium thiosulfate has become the mainstay of treatment. Parathyroidectomy is a recommended modality of treatment in those with high PTH levels. Our case was unique in that calciphylaxis developed after subtotal parathyroidectomy. We believe that this is due to a decreased PTH level and decreasing bone turnover which resulted in more circulating calcium facilitating vascular and soft tissue calcification. The exact mechanism of developing calciphylaxis after parathyroidectomy is unknown. Even though parathyroidectomy is an effective treatment for calciphylaxis, clinicians should be aware that it can rarely present after parathyroidectomy.

Persistence of Vascular Calcification after Reversal of Uremia

The extent to which vascular calcification is reversible and the possible mechanisms are unclear. To address this, calcified aortas from uremic mice were transplanted orthotopically into normal mice, and the calcium content, histology, and minerals of the allografts were compared with the nontransplanted donor aorta. Calcium content decreased immediately after transplantation but remained constant thereafter, with 68% ± 12% remaining after 34 weeks. X-ray diffraction showed the presence of apatite in both donor aortas and allografts. Osteoclasts were absent in the allografts and there was no expression of the macrophage marker CD11b, the osteoclast marker tartrate-resistant acid phosphatase, or carbonic anhydrase II. The initial loss of calcium was less in heavily calcified aortas and was associated with an increase in the Ca/P ratio from 1.49 to 1.63, consistent with a loss of nonapatitic calcium. The results indicate that vascular calcification persists after reversal of uremia, because of a lack of active resorption of apatite. This failure to resorb established calcifications may contribute to the severity of vascular calcification and suggests that therapy should be aimed at prevention.

The association of bone and osteoclasts with vascular calcification

The presence of bone tissue in calcified arteries may provide insights into the pathophysiology and potential reversibility of calcification, but the prevalence, distribution, and determinants of bone and osteoclasts in calcified arteries are unknown. Specimens of 386 arteries from lower limb amputations in 108 patients were examined retrospectively. Calcification was present in 282 arteries from 89 patients, which was medial in 64%, intimal in 9%, and both in 27%. Bone was present in 6% of arteries, essentially all of which were heavily calcified. Multiple sampling revealed that the true prevalence of bone in heavily calcified arteries was 25%. Bone was more common in medial rather than intimal calcifications (10% vs 3%, p=0.03) but did not vary with artery location (above vs below the knee). Heavily calcified arteries with bone were more likely to come from patients who were older (p=0.04), had diabetes (p=0.06), or were receiving warfarin (p=0.06), but there was no association with gender or renal failure. Bone was almost always adjacent to calcifications, along the periphery, but never within. Staining for the bone-specific proteins osteocalcin and osterix was noted in 20% and 45% of heavily calcified arteries without visible bone. Osteoclasts were present in 4.9% of arteries, all of which were heavily calcified and most of which contained bone. The frequent absence of bone in heavily calcified vessels and the histologic pattern strongly suggests a secondary rather than primary event. Recruitment of osteoclasts to vascular calcifications can occur but is rare, suggesting a limited capacity to reverse calcifications.

Vascular Calcification Revisited: A New Perspective for Phosphate Transport

Elevated serum phosphorus has emerged as a key risk factor for pathologic calcification of cardiovascular structures, or vascular calcification (VC). To prevent the formation of calciumphosphate deposits (CPD), the body uses adenosine-5’-triphosphate (ATP) to synthesize inhibitors of calcification, including proteins and inhibitors of low molecular weight. Extracellular pyrophosphate (PPi) is a potent inhibitor of VC, which is produced during extracellular hydrolysis of ATP. Loss of function in the enzymes and transporters that are involved in the cycle of extracellular ATP, including Pi transporters, leads to excessive deposition of calcium-phosphate salts. Treatment of hyperphosphatemia with Pi-binders and Injection of exogenous PPi are the effective treatments to prevent CPD in the aortic wall. The role of sodium phosphate cotransporters in ectopic calcification is contradictory and not well defined, but their important role in the control of intracellular Pi levels and the synthesis of ATP make them an important target to study.

Dysregulation of phosphate metabolism and conditions associated with phosphate toxicity

Phosphate homeostasis is coordinated and regulated by complex cross-organ talk through delicate hormonal networks. Parathyroid hormone (PTH), secreted in response to low serum calcium, has an important role in maintaining phosphate homeostasis by influencing renal synthesis of 1,25-dihydroxyvitamin D, thereby increasing intestinal phosphate absorption. Moreover, PTH can increase phosphate efflux from bone and contribute to renal phosphate homeostasis through phosphaturic effects. In addition, PTH can induce skeletal synthesis of another potent phosphaturic hormone, fibroblast growth factor 23 (FGF23), which is able to inhibit renal tubular phosphate reabsorption, thereby increasing urinary phosphate excretion. FGF23 can also fine-tune vitamin D homeostasis by suppressing renal expression of 1-alpha hydroxylase (1α(OH)ase). This review briefly discusses how FGF23, by forming a bone-kidney axis, regulates phosphate homeostasis, and how its dysregulation can lead to phosphate toxicity that induces widespread tissue injury. We also provide evidence to explain how phosphate toxicity related to dietary phosphorus overload may facilitate incidence of noncommunicable diseases including kidney disease, cardiovascular disease, cancers and skeletal disorders.

Arterial microcalcification in atherosclerotic patients with and without chronic kidney disease: a comparative high-resolution scanning X-ray diffraction analysis

Vascular calcification, albeit heterogeneous in terms of biological and physicochemical properties, has been associated with ageing, lifestyle, diabetes, and chronic kidney disease (CKD). It is unknown whether or not moderately impaired renal function (CKD stages 2-4) affects the physiochemical composition and/or the formation of magnesium-containing tricalcium phosphate ([Ca,Mg](3)[PO(4)](2), whitlockite) in arterial microcalcification. Therefore, a high-resolution scanning X-ray diffraction analysis (European Synchrotron Radiation Facility, Grenoble, France) utilizing histological sections of paraffin-embedded arterial specimens derived from atherosclerotic patients with normal renal function (n = 15) and CKD (stages 2-4, n = 13) was performed. This approach allowed us to spatially assess the contribution of calcium phosphate (apatite) and whitlockite to arterial microcalcification. Per group, the number of samples (13 vs. 12) with sufficient signal intensity and total lengths of regions (201 vs. 232 μm) giving rise to diffractograms ("informative regions") were comparable. Summarizing all informative regions per group into one composite sample revealed calcium phosphate/apatite as the leading mineral phase in CKD patients, whereas in patients with normal renal function the relative contribution of whitlockite and calcium phosphate/apatite was on the same order of magnitude (CKD, calcium phosphate/apatite 157 μm, whitlockite 38.7 μm; non-CKD, calcium phosphate/apatite 79.0 μm, whitlockite 94.1 μm; each p < 0.05). Our results, although based on a limited number of samples, indicate that chronic impairment of renal function affects local magnesium homeostasis and thus contributes to the physicochemical composition of microcalcification in atherosclerotic patients.

Calcium-sensing receptor, calcimimetics, and cardiovascular calcifications in chronic kidney disease

Renal function impairment goes along with a disturbed calcium, phosphate, and vitamin D metabolism, resulting in secondary hyperparathyroidism (sHPT). These mineral metabolism disturbances are associated with soft tissue calcifications, particularly arteries, cardiac valves, and myocardium, ultimately associated with increased risk of mortality in patients with chronic kidney disease (CKD). sHPT may lead to cardiovascular calcifications by other mechanisms including an impaired effect of parathyroid hormone (PTH), and a decreased calcium-sensing receptor (CaR) expression on cardiovascular structures. PTH may play a direct role on vascular calcifications through activation of a receptor, the type-1 PTH/PTHrP receptor, normally attributed to PTH-related peptide (PTHrP). The CaR in vascular cells may also play a role on vascular mineralization as suggested by its extremely reduced expression in atherosclerotic calcified human arteries. Calcimimetic compounds increasing the CaR sensitivity to extracellular calcium efficiently reduce serum PTH, calcium, and phosphate in dialysis patients with sHPT. They upregulate the CaR in vascular cells and attenuate vascular mineralization in uremic states. In this article, the pathophysiological mechanisms associated with cardiovascular calcifications in case of sHPT, the impact of medical and surgical correction of sHPT, the biology of the CaR in vascular structures and its function in CKD state, and finally the role played by the CaR and its modulation by the calcimimetics on uremic-related cardiovascular calcifications are reviewed.

Recent progress in the treatment of vascular calcification

Vascular calcification is common in patients with advanced chronic kidney disease and is associated with poorer outcomes. Although the pathophysiology is not completely understood, it is clear that it is a multifactorial process involving altered mineral metabolism, as well as changes in systemic and local factors that can promote or inhibit vascular calcification, and all of these are potential therapeutic targets. Current therapy is closely linked to strategies for preventing disordered bone and mineral metabolism in advanced kidney disease and involves lowering the circulating levels of both phosphate and calcium. The efficacy of compounds that specifically target calcification, such as bisphosphonates and thiosulfate, has been shown in animals but only in small numbers of humans, and safety remains an issue. Additional therapies, such as pyrophosphate, vitamin K, and lowering of pH, are supported by animal studies, but are yet to be investigated clinically. As the mineral composition of vascular calcifications is the same as in bone, potential effects on bone must be addressed with any therapy for vascular calcification.

Phosphate: despite advances in research, the benefits to patients remain limited

Tremendous progress has been made over recent years in the understanding of the mechanisms regulating calcium, phosphorus, vitamin D, and parathyroid hormone homeostasis in the normal human body and in patients with different degrees of renal failure. In addition, some of these findings have been translated into clinical practice, be it diagnostic or therapeutic.

Autocrine ATP release coupled to extracellular pyrophosphate accumulation in vascular smooth muscle cells

Extracellular inorganic pyrophosphate (PP(i)) is a potent suppressor of physiological calcification in bone and pathological calcification in blood vessels. Ectonucleotide pyrophosphatase/phosphodiesterases (eNPPs) generate PP(i) via the hydrolysis of ATP released into extracellular compartments by poorly understood mechanisms. Here we report that cultured vascular smooth muscle cells (VSMC) from rat aorta generate extracellular PP(i) via an autocrine mechanism that involves ATP release tightly coupled to eNPP activity. The nucleotide analog beta,gamma-methylene ATP (MeATP or AMPPCP) was used to selectively suppress ATP metabolism by eNPPs but not the CD39-type ecto-ATPases. In the absence of MeATP, VSMC generated extracellular PP(i) to accumulate >or=600 nM within 2 h while steadily maintaining extracellular ATP at 1 nM. Conversely, the presence of MeATP completely suppressed PP(i) accumulation while increasing ATP accumulation. Probenecid, which inhibits PP(i) efflux dependent on ANK, a putative PP(i) transporter or transport regulator, reduced extracellular PP(i) accumulation by approximately twofold. This indicates that autocrine ATP release coupled to eNPP activity comprises >or=50% of the extracellular PP(i)-generating capacity of VSMC. The accumulation of extracellular PP(i) and ATP was markedly attenuated by reduced temperature but was insensitive to brefeldin A, which suppresses constitutive exocytosis of Golgi-derived secretory vesicles. The magnitude of extracellular PP(i) accumulation in VSMC cultures increased with time postplating, suggesting that ATP release coupled to PP(i) generation is upregulated as cultured VSMC undergo contact-inhibition of proliferation or deposit extracellular matrix.

Effect of bisphosphonates on vascular calcification and bone metabolism in experimental renal failure

Although it is known that bisphosphonates prevent medial vascular calcification in vivo, their mechanism of action remains unknown and, in particular, whether they act directly on the blood vessels or indirectly through inhibition of bone resorption. To determine this, we studied the effects of two bisphosphonates on calcification of rat aortas in vitro and on in vivo aortic calcification and bone metabolism in rats with renal failure. We produced vascular calcification in rats with adenine-induced renal failure fed a high-phosphate diet. Daily treatment with either etidronate or pamidronate prevented aortic calcification, with the latter being 100-fold more potent. Both aortic calcification and bone formation were reduced in parallel; however, bone resorption was not significantly affected. In all uremic rats, aortic calcium content correlated with bone formation but not with bone resorption. Bisphosphonates also inhibited calcification of rat aortas in culture and arrested further calcification of precalcified vessels but did not reverse their calcification. Expression of osteogenic factors or calcification inhibitors was not altered by etidronate in vitro. Hence, these studies show that bisphosphonates can directly inhibit uremic vascular calcification independent of bone resorption. The correlation between inhibition of aortic calcification and bone mineralization is consistent with a common mechanism such as the prevention of hydroxyapatite formation and suggests that bisphosphonates may not be able to prevent vascular calcification without inhibiting bone formation in uremic rats.

Dermal inorganic gadolinium concentrations: evidence for in vivo transmetallation and long-term persistence in nephrogenic systemic fibrosis

BACKGROUND

Gadolinium (Gd)-based magnetic resonance contrast agents (GBMCA), including gadodiamide, have been identified as the probable causative agents of the serious disease, nephrogenic systemic fibrosis (NSF).

OBJECTIVES

To investigate retained Gd-containing deposits in skin biopsies from patients with NSF and to determine their relative concentrations over time from administration of GBMCA.

METHODS

An investigator-blinded retrospective study, analysing 43 skin biopsies from 20 patients with gadodiamide-related NSF and one NSF-negative gadodiamide-exposed dialysis patient, ranging from 16 days to 1991 days after Gd contrast dose. Utilizing automated quantitative scanning electron microscopy/energy-dispersive X-ray spectroscopy we determined the concentration of Gd and associated elements present as insoluble deposits in situ in the tissues.

RESULTS

We detected Gd in skin lesions of all 20 patients with NSF, whereas Gd was undetectable in the NSF-negative patient. Gd concentration increased over time in 60% of patients with multiple sequential biopsies (n=10), decreasing only when the initial sampling time was >23 months after first gadodiamide dose. All Gd-containing deposits contained phosphorus, calcium and sodium. The ratio of Gd to calcium in tissue deposits correlated positively with the gadodiamide dose and with serum ionized calcium at the time of Gd exposure.

CONCLUSIONS

These findings demonstrate the in vivo release (through transmetallation) of the toxic free Gd3+ from gadodiamide, and its retention in apatite-like deposits. We suggest that Gd may be mobilized over time from bone stores, explaining variably delayed onset of NSF and increasing skin concentration over time in patients with NSF.

Long-term retention of gadolinium in tissues from nephrogenic systemic fibrosis patient after multiple gadolinium-enhanced MRI scans: case report and implications

Nephrogenic systemic fibrosis (NSF) is a painful and debilitating fibrosing disorder of the skin and systemic tissues. It is associated with exposure to Gd, used in MRIs and MRAs, in patients with renal insufficiency. We here present an illustrative example of a young patient who underwent multiple Gd-enhanced scans, both before and after developing severe NSF. We examined biopsy tissues for quantification of detectable insoluble Gd deposits using automated scanning electron microscopy/energy dispersive X-ray spectroscopy. High concentrations of Gd associated with calcium and phosphorus in skin persisted even 3 years after the last exposure to Gd. Such long-term retention of Gd raises further concerns about the utility and safety of Gd-based contrast agents. Residual Gd chelates, after initial and rapid renal clearance, can dissociate into insoluble, toxic Gd(3+) that precipitates with tissue anions. Bone serves as a site for Gd storage. Subsequent clearance and mobilization from such stores may explain the variable latency of onset of NSF. We hypothesize that long-term persistence and slow release of Gd(3+) from bone stores can be a cause for concern of Gd-associated toxicity with long latency.