Anti-tumor Activity of the Type I PRMT Inhibitor, GSK3368715, Synergizes with PRMT5 Inhibition through MTAP Loss

Type I protein arginine methyltransferases (PRMTs) catalyze asymmetric dimethylation of arginines on proteins. Type I PRMTs and their substrates have been implicated in human cancers, suggesting inhibition of type I PRMTs may offer a therapeutic approach for oncology. The current report describes GSK3368715 (EPZ019997), a potent, reversible type I PRMT inhibitor with anti-tumor effects in human cancer models. Inhibition of PRMT5, the predominant type II PRMT, produces synergistic cancer cell growth inhibition when combined with GSK3368715. Interestingly, deletion of the methylthioadenosine phosphorylase gene (MTAP) results in accumulation of the metabolite 2-methylthioadenosine, an endogenous inhibitor of PRMT5, and correlates with sensitivity to GSK3368715 in cell lines. These data provide rationale to explore MTAP status as a biomarker strategy for patient selection.

Reductive carboxylation supports redox homeostasis during anchorage-independent growth

Cells receive growth and survival stimuli through their attachment to an extracellular matrix (ECM). Overcoming the addiction to ECM-induced signals is required for anchorage-independent growth, a property of most malignant cells. Detachment from ECM is associated with enhanced production of reactive oxygen species (ROS) owing to altered glucose metabolism. Here we identify an unconventional pathway that supports redox homeostasis and growth during adaptation to anchorage independence. We observed that detachment from monolayer culture and growth as anchorage-independent tumour spheroids was accompanied by changes in both glucose and glutamine metabolism. Specifically, oxidation of both nutrients was suppressed in spheroids, whereas reductive formation of citrate from glutamine was enhanced. Reductive glutamine metabolism was highly dependent on cytosolic isocitrate dehydrogenase-1 (IDH1), because the activity was suppressed in cells homozygous null for IDH1 or treated with an IDH1 inhibitor. This activity occurred in absence of hypoxia, a well-known inducer of reductive metabolism. Rather, IDH1 mitigated mitochondrial ROS in spheroids, and suppressing IDH1 reduced spheroid growth through a mechanism requiring mitochondrial ROS. Isotope tracing revealed that in spheroids, isocitrate/citrate produced reductively in the cytosol could enter the mitochondria and participate in oxidative metabolism, including oxidation by IDH2. This generates NADPH in the mitochondria, enabling cells to mitigate mitochondrial ROS and maximize growth. Neither IDH1 nor IDH2 was necessary for monolayer growth, but deleting either one enhanced mitochondrial ROS and reduced spheroid size, as did deletion of the mitochondrial citrate transporter protein. Together, the data indicate that adaptation to anchorage independence requires a fundamental change in citrate metabolism, initiated by IDH1-dependent reductive carboxylation and culminating in suppression of mitochondrial ROS.

Abstract A35: Cytosolic reductive carboxylation is required for mitochondrial redox homeostasis during anchorage-independent cell growth

Epithelial cells receive growth and survival stimuli through their attachment to an extracellular matrix (ECM)1. Overcoming the addiction to ECM-induced signals is required for anchorage-independent growth, an essential hallmark of cells capable of metastasis2. Previous study showed that detachment from the ECM is associated with enhanced reactive oxygen species (ROS) levels due to suppression of glucose metabolism in the pentose phosphate pathway3. Here we used metabolic flux analysis to identify an unconventional metabolic pathway that supports redox homeostasis and growth during adaptation to anchorage independence. We observed that detachment from monolayer culture and growth as anchorage-independent tumor spheroids was accompanied by changes in both glucose and glutamine metabolism. Specifically, oxidative metabolism of both glucose and glutamine was suppressed in the spheroids, whereas reductive formation of citrate from glutamine was enhanced. Enhanced reductive glutamine metabolism was highly dependent on cytosolic isocitrate dehydrogenase-1 (IDH1) rather than mitochondrial IDH2, because this activity was eliminated in cells homozygous null for IDH1 or treated with an IDH1 inhibitor. Reductive carboxylation occurred in the absence of hypoxia, a well-known inducer of reductive glutamine metabolism4, 5. IDH1dependent reductive carboxylation mitigated mitochondrial ROS during spheroid growth, and IDH1 deletion or inhibition blunted spheroid growth in a ROS-dependent manner. Ablating expression of IDH2 or the mitochondrial citrate transporter did not substantially alter reductive labeling, but suppressed spheroid growth. Together, the data indicate that adaptation to anchorage independence requires a fundamental change in citrate metabolism. During anchorage-independent culture, reductive carboxylation produces cytosolic citrate, some of which is then imported into the mitochondria and metabolized in the TCA cycle. This results in the net transfer of NADPH from the cytosol to the mitochondria, mitigating mitochondrial ROS and maximizing cell growth.

References:

1. Bhowmick NA, Neilson EG, Moses HL. Stromal fibroblasts in cancer initiation and progression. Nature 2004; 432:332-7.

2. Hanahan D, Weinberg RA. Hallmarks of cancer: the next generation. Cell 2011; 144:646-74.

3. Schafer ZT, Grassian AR, Song L, Jiang Z, Gerhart-Hines Z, Irie HY, Gao S, Puigserver P, Brugge JS. Antioxidant and oncogene rescue of metabolic defects caused by loss of matrix attachment. Nature 2009; 461:109-13.

4. Wise DR, Ward PS, Shay JE, Cross JR, Gruber JJ, Sachdeva UM, Platt JM, DeMatteo RG, Simon MC, Thompson CB. Hypoxia promotes isocitrate dehydrogenase-dependent carboxylation of alpha-ketoglutarate to citrate to support cell growth and viability. Proceedings of the National Academy of Sciences of the United States of America 2011; 108:19611-6.

5. Metallo CM, Gameiro PA, Bell EL, Mattaini KR, Yang J, Hiller K, Jewell CM, Johnson ZR, Irvine DJ, Guarente L, et al. Reductive glutamine metabolism by IDH1 mediates lipogenesis under hypoxia. Nature 2012; 481:380-4.

Citation Format: Lei Jiang, Alexander Shestov, Lance S. Terada, Nicholas D. Adams, Michael T. McCabe, Beth Pietrak, Stan J. Schimidt, Benjamin Schwartz, Ralph J. DeBerardinis. Cytosolic reductive carboxylation is required for mitochondrial redox homeostasis during anchorage-independent cell growth. [abstract]. In: Proceedings of the AACR Special Conference: Metabolism and Cancer; Jun 7-10, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(1_Suppl):Abstract nr A35.

Abstract C38: Novel allosteric IDH1 mutant Inhibitors for differentiation therapy of acute myeloid leukemia

Mutations in the isocitrate dehydrogenase 1 (IDH1) gene are known driver mutations in acute myeloid leukemia (AML) and other cancer types. AML is hallmarked by a differentiation block and patient outcomes remain poor, especially for patients above 60 years of age who typically do not tolerate high dose chemotherapy and stem cell transplantation, leading to cure rates below 20%. Hence the development of novel targeted therapies for treatment of AML subtypes are required. Of note, inhibitors of mutants of the closely related IDH2 gene as well as IDH1 have recently been described and show promising pre-clinical and early phase clinical activity. However, the specific molecular and functional effects of IDH1 inhibitors in AML, including in primary patients' cells, have not been reported yet.

Here, we report the development of novel allosteric inhibitors of mutant IDH1 for differentiation therapy of acute myeloid leukemia. A high-throughput biochemical screen targeting an IDH1 heterodimer composed of R132H and WT IDH1 led to the identification of a tetrahydropyrazolopyridine series of inhibitors. Structural and biochemical analyses revealed that these novel compounds bind to an allosteric site that does not contact any of the mutant residues in the enzymes active site and inhibit enzymatic turnover. The enzyme complex locked in the catalytically inactive conformation inhibits the production of the oncometabolite 2-hydroxyglutarate (2-HG). In biochemical studies, we observed potent inhibition of several different clinically relevant R132 mutants in the presence or absence of the cofactor NADPH, accompanied by significant decrease in H3K9me2 levels.

Treatment of primary IDH1 mutant AML patients' cells ex vivo uniformly led to a decrease in intracellular 2-HG, abrogation of the myeloid differentiation block, increased cell death and induction of differentiation both at the level of leukemic blasts and immature stem-like cells. Allosteric inhibition of IDH1 also led to a decrease in leukemic blasts in an in vivo xenotransplantation model. At the molecular level, enhanced reduced representation bisulfite sequencing showed that treatment with allosteric IDH1 inhibitors led to a significant reversal of the DNA cytosine hypermethylation pattern induced by mutant IDH1, accompanied by gene expression changes of key sets of genes and pathways, including “Cell Cycle”, “G1/S transition”, “Cellular growth and proliferation”, and “Cell death and survival”.

Taken together, our findings provide novel insight into the effects of inhibition of mutant IDH1 in primary AML patients' cells and open avenues for future investigations with these and other novel allosteric inhibitors for targeting IDH1 mutants in leukemia and possibly in other cancers.

Citation Format: Ujunwa C. Okoye-Okafor, Boris Bartholdy, Jessy Cartier, Enoch Gao, Beth Pietrak, Alan R. Rendina, Cynthia Rominger, Chad Quinn, Angela Smallwood, Ken Wiggall, Alexander Reif, Stan Schmidt, Hongwei Qi, Huizhen Zhao, Gerard Joberty, Maria Faelth-Savitski, Marcus Bantscheff, Gerard Drewes, Chaya Duraiswami, Pat Brady, Swathi-Rao Narayanagari, Ileana Antony-Debre, Kelly Mitchell, Heng Rui Wang, Yun-Ruei Kao, Maximilian Christopeit, Luis Carvajal, Laura Barreyro, Elisabeth Paietta, Britta Will, Nestor Concha, Nicholas D. Adams, Benjamin Schwartz, Michael T. McCabe, Jaroslav Maciejewski, Amit Verma, Ulrich Steidl. Novel allosteric IDH1 mutant Inhibitors for differentiation therapy of acute myeloid leukemia. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr C38.

New IDH1 mutant inhibitors for treatment of acute myeloid leukemia

Neomorphic mutations in isocitrate dehydrogenase 1 (IDH1) are driver mutations in acute myeloid leukemia (AML) and other cancers. We report the development of new allosteric inhibitors of mutant IDH1. Crystallographic and biochemical results demonstrated that compounds of this chemical series bind to an allosteric site and lock the enzyme in a catalytically inactive conformation, thereby enabling inhibition of different clinically relevant IDH1 mutants. Treatment of IDH1 mutant primary AML cells uniformly led to a decrease in intracellular 2-HG, abrogation of the myeloid differentiation block and induction of granulocytic differentiation at the level of leukemic blasts and more immature stem-like cells, in vitro and in vivo. Molecularly, treatment with the inhibitors led to a reversal of the DNA cytosine hypermethylation patterns caused by mutant IDH1 in the cells of individuals with AML. Our study provides proof of concept for the molecular and biological activity of novel allosteric inhibitors for targeting different mutant forms of IDH1 in leukemia.

Discovery of a Potent Class of PI3Kα Inhibitors with Unique Binding Mode via Encoded Library Technology (ELT)

In the search of PI3K p110α wild type and H1047R mutant selective small molecule leads, an encoded library technology (ELT) campaign against the desired target proteins was performed which led to the discovery of a selective chemotype for PI3K isoforms from a three-cycle DNA encoded library. An X-ray crystal structure of a representative inhibitor from this chemotype demonstrated a unique binding mode in the p110α protein.

Mutant IDH1 enhances the production of 2-hydroxyglutarate due to its kinetic mechanism

The human, cytosolic enzyme isocitrate dehydrogenase 1 (IDH1) reversibly converts isocitrate to α-ketoglutarate (αKG). Cancer-associated somatic mutations in IDH1 result in a loss of this normal function but a gain in a new or neomorphic ability to convert αKG to the oncometabolite 2-hydroxyglutarate (2HG). To improve our understanding of the basis for this phenomenon, we have conducted a detailed kinetic study of wild-type IDH1 as well as the known 2HG-producing clinical R132H and G97D mutants and mechanistic Y139D and (newly described) G97N mutants. In the reductive direction of the normal reaction (αKG to isocitrate), dead-end inhibition studies suggest that wild-type IDH1 goes through a random sequential mechanism, similar to previous reports on related mammalian IDH enzymes. However, analogous experiments studying the reductive neomorphic reaction (αKG to 2HG) with the mutant forms of IDH1 are more consistent with an ordered sequential mechanism, with NADPH binding before αKG. This result was further confirmed by primary kinetic isotope effects for which saturating with αKG greatly reduced the observed isotope effect on (D)(V/K)NADPH. For the mutant IDH1 enzyme, the change in mechanism was consistently associated with reduced efficiencies in the use of αKG as a substrate and enhanced efficiencies using NADPH as a substrate. We propose that the sum of these kinetic changes allows the mutant IDH1 enzymes to reductively trap αKG directly into 2HG, rather than allowing it to react with carbon dioxide and form isocitrate, as occurs in the wild-type enzyme.

Discovery of GSK2126458, a Highly Potent Inhibitor of PI3K and the Mammalian Target of Rapamycin

Phosphoinositide 3-kinase α (PI3Kα) is a critical regulator of cell growth and transformation, and its signaling pathway is the most commonly mutated pathway in human cancers. The mammalian target of rapamycin (mTOR), a class IV PI3K protein kinase, is also a central regulator of cell growth, and mTOR inhibitors are believed to augment the antiproliferative efficacy of PI3K/AKT pathway inhibition. 2,4-Difluoro-N-{2-(methyloxy)-5-[4-(4-pyridazinyl)-6-quinolinyl]-3-pyridinyl}benzenesulfonamide (GSK2126458, 1) has been identified as a highly potent, orally bioavailable inhibitor of PI3Kα and mTOR with in vivo activity in both pharmacodynamic and tumor growth efficacy models. Compound 1 is currently being evaluated in human clinical trials for the treatment of cancer.

Discovery of the First Potent and Selective Inhibitor of Centromere-Associated Protein E: GSK923295

Inhibition of mitotic kinesins represents a novel approach for the discovery of a new generation of anti-mitotic cancer chemotherapeutics. We report here the discovery of the first potent and selective inhibitor of centromere-associated protein E (CENP-E) 3-chloro-N-{(1S)-2-[(N,N-dimethylglycyl)amino]-1-[(4-{8-[(1S)-1-hydroxyethyl]imidazo[1,2-a]pyridin-2-yl}phenyl)methyl]ethyl}-4-[(1-methylethyl)oxy]benzamide (GSK923295; 1), starting from a high-throughput screening hit, 3-chloro-4-isopropoxybenzoic acid 2. Compound 1 has demonstrated broad antitumor activity in vivo and is currently in human clinical trials.

Novel ATP-competitive kinesin spindle protein inhibitors

Kinesin spindle protein (KSP), an ATPase responsible for spindle pole separation during mitosis that is present only in proliferating cells, has become a novel and attractive anticancer target with potential for reduced side effects compared to currently available therapies. We report herein the discovery of the first known ATP-competitive inhibitors of KSP, which display a unique activity profile as compared to the known loop 5 (L5) allosteric KSP inhibitors that are currently under clinical evaluation. Optimization of this series led to the identification of biphenyl sulfamide 20, a potent KSP inhibitor with in vitro antiproliferative activity against human cells with either wild-type KSP (HCT116) or mutant KSP (HCT116 D130V). In a murine xenograft model with HCT116 D130V tumors, 20 showed significant antitumor activity following intraperitoneal dosing, providing in vivo proof-of-principle of the efficacy of an ATP-competitive KSP inhibitor versus tumors that are resistant to the other known KSP inhibitors.

Total synthesis of bafilomycin V(1): a methanolysis product of the macrolide bafilomycin C(2)

A synthesis of bafilomycin V(1), a methanolysis product of the macrolide natural product bafilomycin C(2), is described. The route utilizes chiral nonracemic allenylzinc reagents, prepared in situ from propargylic mesylates, to access key segments of this methyl ester. The acetylenic moieties of the derived homopropargylic alcohol adducts play an important role in further elaboration of these subunits. Final assemblage of the 25-carbon chain, containing 12 stereocenters, an alpha-methoxy Z,E 1,3-dienic ester, and an additional E,E 1,3-diene, was achieved through Stille coupling of an acetylene-derived vinyl stannane and vinyl iodide of approximate equal complexity. Attempted cyclization of several C15 hydroxy acid derivatives to the 16-membered lactone bafilomycin A(1), a potent inhibitor of vacuolar ATPases, could not be achieved.

Shunt nephritis from Propionibacterium acnes in a solitary kidney

Since its initial description in 1965, immune complex glomerulonephritis associated with ventriculoatrial shunts (VAS) has been reported widely in the literature. The most common incriminating organism is Staphylococcus epidermidis, but less often, an organism generally regarded as nonpathogenic, such as Propionibacterium acnes, has been noted as the cause. Shunt infection usually occurs within a few months after placement or manipulation of the shunt, and shunt nephritis (SN) develops gradually over months to years after. Treatment involves mandatory removal of the shunt and antibiotics; prognosis is variable. We report a case of SN with P acnes that is unusual because of its occurrence in a solitary kidney 6 years after shunt placement, persistently negative blood cultures, and normal complement levels. Percutaneous biopsy of a solitary kidney should be considered if it is expected that the result may guide therapy of progressive renal failure.

Progress toward the total synthesis of bafilomycin A(1): stereoselective synthesis of the C15-C25 subunit by additions of nonracemic allenylzinc reagents to aldehydes

[reaction: see text] A highly stereoselective synthesis of the C15-C25 subunit (2) of bafilomycin A(1) (1) has been accomplished by a route utilizing additions of chiral nonracemic allenylzinc reagents to aldehydes.

Acid and mineral balances and bone in familial proximal renal tubular acidosis

BACKGROUND

Metabolic acidosis caused by increased rates of fixed acid production is associated with increased urinary excretion of Ca and negative Ca balances. Metabolic acidosis caused by a reduced capacity of the kidneys to excrete acid contributes to the development of bone disease in the course of chronic renal failure and may be associated with bone disease among some patients with renal tubular acidosis.

METHODS

To assess the effects of life-long metabolic acidosis alone in the absence of other physiological disturbances, we measured the net balances of fixed acid and minerals in two brothers in a Costa Rican family with hereditary proximal renal tubular acidosis. Bone radiographs were assessed, and radial bone densities were measured. On a subsequent occasion, transiliac bone biopsies, following double-tetracycline labeling, were obtained from these two patients and an unaffected brother.

RESULTS

During the balance studies, serum [HCO3-] concentrations of the two affected patients were stable at 12.5 +/- 0.9 and 19.2 +/- 0.7 mmol/L, respectively. Their rates of net fixed acid production were normal and appropriate for their body weights, averaging 0.90 and 1.02 mEq/kg/day. Because their distal renal tubular function was normal, they were capable of acidifying their urine maximally, allowing sufficient urinary excretion of titratable acid and ammonium to maintain net acid excretion at a level that matched acid production. Thus, their acid balances were near zero, as observed among healthy subjects, at -1.9 +/- 2.3 and -2.2 +/- 2.2 mEq/day, respectively. Their rates of urinary Ca excretion were normal at 1.6 +/- 0.3 and 2.7 +/- 2.4 mmol/day, and the their balances of Ca and other minerals were close to zero so that ongoing bone loss was not occurring despite the acidosis. Nevertheless, their heights, relative to their ages, were shorter than the heights of their unaffected relatives. Their radial bone densities were lower than normal for their age and sex, and their iliac cortices were thinner than that of their unaffected brother. However, they had no histomorphometric evidence of osteomalacia or osteitis fibrosa, and their rates of bone mineralization were normal.

CONCLUSIONS

The results indicate that this chronic metabolic acidosis reduces growth, including that of bone. We speculate, without direct supporting evidence, that bone stores of HCO3-/CO3= are reduced, as has been observed in patients with the metabolic acidosis of chronic renal failure and in experimental metabolic acidosis in animals.

Nephrocalcinosis

The pathogenesis of NC in VLBW infants appears to be multifactorial. The vulnerability of extreme immaturity and the underdevelopment of renal function may be the most important variables. In some ways, we view this problem as similar to that of retinopathy of prematurity. (Clearly the exposure of the retina to high partial pressures of oxygen contributes to the development of retinopathy of prematurity but other variables--some known, such as an immature retina, and others not yet defined--must be present.) Hypercalciuria is common in the VLBW infant, yet not all develop NC. Decreased glomerular filtration rate, low citrate excretion, and frequently an alkaline urine are in part due to the immaturity of renal function of these infants. The need for prolonged hyperalimentation resulting in increased oxalate excretion and the development of BPD frequently requiring diuretics that may cause phosphaturia and magnesium depletion and that may increase calcium excretion are more common in the smallest and sickest of premature infants. Even transient insults to the kidneys, such as hypoxia or hypotension or the use of nephrotoxic drugs that provoke tubular injury and cell death with the probability of crystal formation and growth by way of heterogeneous nucleation, are likely to occur more frequently in this vulnerable population.

Effect of high calcium and phosphorus intake on mineral retention in very low birth weight infants chronically treated with furosemide

The treatment of premature infants with the diuretic furosemide appears to be a contributory factor in the development of metabolic bone disease presumably because of furosemide-induced hypercalciuria. In this study, we measured calcium and phosphorus balance in furosemide-treated very low birth weight infants (VLBW) infants with bronchopulmonary dysplasia (BPD) who were fed a specialized premature formula containing increased amounts of calcium and phosphorus. Furosemide-treated infants received 166 +/- 37 mg/kg/day and retained 80 +/- 34 mg/kg/day of calcium, and 87 +/- 19 mg/kg/day and retained 52 +/- 14 mg/kg/day of phosphorus. The amounts retained were approximately 65% of the calcium and 72% of the phosphorus requirements for in utero mineral accretion. Compared to a group of similarly fed VLBW infants without BPD and not treated with the diuretic, the furosemide-treated infants excreted a larger percent of the calcium intake in the urine but had similar total urinary calcium and phosphorus losses (mg/kg/day) and serum calcium, phosphorus, alkaline phosphatase, and parathyroid hormone (PTH) levels. From the latter two findings, we suggest that the extra mineral content of the formula may have promoted bone mineralization and prevented the occurrence of secondary hyperparathyroidism.

Absence of anion gap metabolic acidosis in severe methanol poisoning: a case report and review of the literature

Methanol poisoning in humans is characterized by a latent period with subsequent development of anion gap metabolic acidosis and blindness. We describe a patient with potentially lethal methanol ingestion as evidenced by an admission serum methanol level of 403 mg/dL and sustained serum methanol levels greater than 50 mg/dL for more than 18 hours after ingestion, despite hemodialysis therapy. That anion gap metabolic acidosis or visual impairment did not develop in this patient was attributed to documented prior ethanol ingestion (admission serum ethanol level of 158 mg/dL) and continued ethanol administration during hospitalization (sustained serum ethanol levels greater than 100 mg/dL). This case demonstrates the ability of ethanol to inhibit the metabolism of methanol to formic acid in humans. This inhibition was achieved without induction of lactic acidosis. Thus this case documents the efficacy of ethanol therapy in patients with methanol poisoning.

Review: Renal osteodystrophy--pathogenesis and treatment

Histologic bone changes of osteitis fibrosa and osteomalacia are commonly present in patients with end-stage renal disease. Although many patients are not symptomatic from these bone changes, some patients are severely disabled. Altered metabolism of vitamin D, calcium, phosphorus, and parathyroid hormone occurs in renal failure and contributes to the development of uremic bone disease. This article reviews the current theories of pathogenesis and treatment of renal osteodystrophy. In addition, the clinical presentation, pathogenesis, and treatment of the various aluminum-associated osteomalacic syndromes in uremia are discussed.

Renal osteodystrophy--pathogenesis and treatment

Histologic bone changes of osteitis fibrosa and osteomalacia are commonly present in patients with end-stage renal disease. Although many patients are not symptomatic from these bone changes, some patients are severely disabled. Altered metabolism of vitamin D, calcium, phosphorus, and parathyroid hormone occurs in renal failure and contributes to the development of uremic bone disease. This article reviews the current theories of pathogenesis and treatment of renal osteodystrophy. In addition, the clinical presentation, pathogenesis, and treatment of the various aluminum-associated osteomalacic syndromes in uremia are discussed.

Effects of weight loss on serum 1,25-(OH)2-vitamin D concentrations in adults: a preliminary report

During a review of 42 metabolic studies in healthy women and men we observed that serum 1,25-(OH)2-D concentrations were directly correlated to the observed daily changes in body weight (r = 0.68; P less than 0.001) and to caloric intake/kg/day (r = 0.39; P = 0.01). These relationships could not be accounted for by related and physiologically expected changes in serum Ca or iPTH concentrations. However, serum 1,25-(OH)2-D concentrations were observed to be inversely correlated to serum PO4 levels (r = -0.44; P = 0.004). In addition, serum PO4 levels were inversely correlated to the daily changes in body weight (r = -0.40; P = 0.009). Since dietary sodium intake averaged 142 mmol/day, it is unlikely that the observed changes in weight were the result of changes in salt and water balance. Thus it seems reasonable to speculate that serum 1,25-(OH)2-D concentrations may vary directly with energy balance, as reflected by changes in body weight. This effect may be mediated by alterations in PO4 metabolism. The accurate assessment of serum 1,25-(OH)2-D levels thus appears to require several measurements over time periods during which body weight is stable.

Vitamin D metabolites and parathyroid hormone in Cushing's syndrome: relationship to calcium and phosphorus homeostasis

We studied the effects of glucocorticoid excess on calcium and phosphorus homeostasis in relation to vitamin D metabolites and parathyroid hormone (PTH) in seven patients with spontaneous ACTH-dependent Cushing's syndrome. Remission of hypercortisolism resulted in a significant increase in tubular reabsorption of phosphate [from 76 +/- 4% to 89 +/- 2% (mean +/- SEM); P less than 0.01] and serum phosphorus (from 3.1 +/- 0.1 to 4.2 +/- 0.2 mg/dl; P less than 0.005). Serum calcium did not change, although there was a reduction in daily urinary calcium excretion from 0.23 +/- 0.02 to 0.107 +/- 0.02 mg calcium/mg creatinine. Serum immunoreactive PTH (iPTH) levels were normal during Cushing's syndrome (34 +/- 5 microleq/ml), but fell significantly after remission to 22 +/- 2 microleq/ml (P less than 0.05). This small decrease in iPTH did not correlate with the improvement of phosphate homeostasis. Plasma 25-hydroxyvitamin D (25OHD) and 1,25-dihydroxyvitamin D [1,25-(OH2)D] concentrations in Cushing's syndrome did not differ from measurements in 97 normal subjects. After treatment, 25OHD did not change, but 1,25-(OH)2D fell in each patient from a mean of 44 to 22 pg/ml (P less than 0.02). 1,25-(OH)2D was inversely correlated with serum phosphorus (r = 0.59; P less than 0.01), but did not correlate with iPTH. The known impairment of intestinal calcium absorption in Cushing's syndrome cannot be attributed to a decrease in the circulating levels of 1,25-(OH)2D. Endogenous hypercortisolism decreases tubular phosphate reabsorption and serum phosphorus, increase tubular phosphate reabsorption and serum phosphorus, increases iPTH, and results in an increase in 1,25-(OH)2D. These events may contribute to the severe loss of bone mass in such patients and may account for the calciuria and phosphaturia of Cushing's syndrome.

The chief medical residency: a description and recommendations

Individuals occupying the position of chief medical resident have the opportunity to influence significantly the quality and tenor of medical education and medical practice. To assess the status of the position, the authors surveyed chief medical residents completing their tenure in 1980. These residents distributed their time among administrative (41 percent), teaching (35 percent), patient care (21 percent), and research (3 percent) activities. They reported that their experience was quite positive. However, overall satisfaction was significantly negatively correlated with percentage of time spent performing administrative tasks. The percentage of time chief medical residents allocate to administration has increased during the past decade, while time spent teaching and delivering patient care has decreased. Based upon this survey, recommendations for improved utilization of the time and talents of these individuals are presented.

Calcium-oxalate-crystal-induced bone disease

A 13-year-old boy with primary hyperoxaluria and a successful renal allograft developed symptomatic bone disease, hypercalcemia, and hypercalciuria. Transiliac bone biopsy revealed calcium oxalate crystals in the marrow within mononuclear phagocytes and multinucleated giant cells. Deep resorption bays were seen adjacent to these crystal-cell aggregates. Serum 1,25-(OH)2-vitamin D (calcitriol) and iPTH concentrations were low or normal. We suggest that hypercalcemia results from macrophage-mediated bone resorption initiated by Ca oxalate crystal deposition.

Effects of calcitriol administration on calcium metabolism healthy men

To evaluate the relationship between daily and fasting urinary calcium excretion and serum 1,25-(OH)2-D concentrations, we studied six healthy men during control and during chronic oral calcitriol administration (0.6, 1.2, or 1.8 nmoles every 6 hours for 6 to 12 days) while they ate normal and low calcium diets (calcium content, 19.2 or 4.2 mmoles/day). Daily urinary calcium excretion was directly related to serum 1,25-(OH)2-D concentrations but increased more (P less than 0.025) while subjects ate the normal calcium diet (slope, 0.081 mmole/day/pmole/liter, r = 0.94) than when eating the low calcium diet (slope, 0.043 mmole/day/pmole/liter; r=0.83). During calcitriol and ingestion of the low calcium diet, daily urinary calcium excretion averaged 7.32 +/- 2.6 mmoles/day, exceeding the dietary calcium intake (P less than 0.02). Moreover, during calcitriol, when serum 1,25-(OH)2-D exceeded 140 pmoles/liter, fasting urinary calcium/creatinine exceeded 0.34 mmole/mmole (the upper limit of normal) on either diet. When serum 1,25-(OH)2-D concentrations are elevated, a high fasting urinary calcium/creatinine or high daily urinary calcium excretion, even on a low calcium diet, is insufficient criteria for the documentation of a renal calcium leak.

Synthesis and metabolic clearance of 1,25-dihydroxyvitamin D as determinants of serum concentrations: a comparison of two methods

We evaluated endogenous renal 1,25-dihydroxyvitamin D (1,25-OH)2D) synthesis by compartmental analysis of the plasma disappearance of injected [3H]1,25-dihydroxyvitamin D3 in 11 subjects with serum 1,25-(OH)2D concentrations varying from 9-154 pM (normal, 89 +/- 25 pM). Estimated renal synthesis ranged from 2-180 pmol/kg . day. Serum 1,25-(OH)2D concentrations in these subjects best fit a log function of renal synthesis: serum 1,25-(OH)2D, pM = -13 + 74 log renal 1,25-(OH)2D production, picomoles per kg/day (r = 0.91). We also evaluated serum 1,25-(OH)2D concentrations in 6 healthy subjects who had been given 0.6, 1.2, or 1.8 nmol calcitriol every 6 h during a period 6--12 days after hormone administration was begun. Steady serum 1,25-(OH)2D concentrations 2, 4, and 6 h after the last calcitriol dose were achieved in proportion to log dose: serum 1,25-(OH)2D, pM = -12 + 103 log 1,25-dihydroxyvitamin D3 dose, picomoles per kg/day (r = 0.94). Estimated 1,25-(OH)2D production rates using these two methods and assuming a normal serum 1,25-(OH)2D concentration of 89 pM range from 10--24 pmol/kg . day or, for a 70-kg subject, 0.6--1.7 nmol/day or 0.25--0.7 microgram/day. Metabolic clearance of 1,25-(OH)2D appears to be accelerated when production rates are increased.

Is there a disorder of phosphate metabolism in idiopathic hypercalciuria?

Hypophosphatemia either as a consequence of secondary hyperparathyroidism or as a consequence of a primary defect in phosphate metabolism appears to be a well established abnormality among subsets of patients with idiopathic hypercalciuria and nephrolithiasis. The detailed biochemical events that lead to hypophosphatemia in those patients who exhibit a primary abnormality of phosphate metabolism remain to be clarified.

The calciuria of increased fixed acid production in humans: evidence against a role for parathyroid hormone and 1,25(OH)2-vitamin D

We measured mineral and acid balances, serum iPTH, urinary cAMP/creatinine, and plasma concentrations of 25OHD and 1,25(OH)2D in 7 healthy adults during control conditions and during increased fixed acid production achieved either by the administration of NH4Cl (N = 3) or by increased dietary protein intake (N = 4). When acid production was increased, the subjects were in positive acid balance and negative Ca balance because of increased urinary Ca excretion. Serum iPTH fell slightly but urinary cAMP and the plasma levels of vitamin D metabolites did not change. W conclude that the accelerated skeletal and urinary losses of Ca that occur when fixed acid production is increased are not contributed to nor compensated for by the parathyroid-vitamin D endocrine systems.

The interrelationships among prolactin, 1,25-dihydroxyvitamin D, and parathyroid hormone in humans

Serum PRL, parathyroid hormone (PTH), and plasma 1,25-dihydroxyvitamin D [1,25(OH)2D]concentrations were measured in 6 women and 2 men with hyperprolactinemia, 6 normal men and 7 normal women, 4 men and 4 women with primary hyperparathyroidism, and 16 men and 4 women with Ca nephrolithiasis. Plasma 1,25(OH)2D and serum parathyroid hormone (PTH) concentrations were normal in the women and men with hyperprolactinemia. In patients with primary hyperparathyroidism and elevated serum PTH, plasma 1,25(OH)2D concentrations were elevated but serum PRL levels were normal. Likewise, serum PRL levels were normal in patients with Ca nephrolithiasis who had significantly elevated plasma, 1,25(OH)2D concentrations and normal serum PTH concentrations. Thus, hyperprolactinemia due to pituitary adenoma or idiopathic hypersecretion is not accompanied but elevated plasma concentrations of 1,25(OH)2D.

The effects of dihydrotachysterol therapy on the measurement of plasma 1,25-(OH)2-vitamin D in humans

Anephric patients have undetectable plasma concentrations of 1,25-(OH)2-D, but when anephric patients are treated with DHT2 (Hytakerol), their plasma contains a substance that is co-purified with and displaces authentic 3H-1,25(OH)2-D3 from its intestinal cytoplasmic receptor. The concentration of this substance in the plasma of anephric patients taking DHT2 is proportional to the administered daily dose of DHT2 per kilogram body weight. When DHT2 therapy is discontinued, the substance disappears from plasma with an average t 1/2 of about 8 days.

Ethanol induced secretion of calcitonin in chronic renal disease

Whisky (25-50 ml) increased plasma levels of immunoreactive calcitonin (iCT) in seventeen of nineteen patients with chronic renal failure. The effect was greater in patients with high levels of iCT than in those with normal levels. Changes in plasma iCT were not related to changes in calcium, phosphate or immunoreactive gastrin, but were inhibited by the prior administration of propranolol.

Physiological and pharmacological aspects of 24,25-dihydroxycholecalciferol in man

The present study describes the response to small oral doses (1--10 microgram/day) of 24,25-DHCC in man. Contrary to expectation, 24,25-DHCC was as potent as 1,25-DHCC in increasing intestinal absorption of calcium both in normal persons and in patients with a variety of disorders of calcium metabolism. Despite this increase in intestinal absorption, plasma and urine calcium did not increase after 24,25-DHCC as they did after 1,25-DHCC. Metabolic balance studies showed calcium balances to increase by 1.6 to 11.5 mmoles/day in 5 of the 6 patients studied. 24,25-DHCC increased intestinal absorption of calcium equally well in anephric patients, suggesting that conversion of 24,25-DHCC to 1,24,25-trihydroxycholecalciferol by the kidney cannot be the sole mechanism by which 24,25-DHCC expresses biological activity, even though in vitamin D deficient rats nephrectomy does abolish the ability of large doses of 24,25-DHCC to increase calcium absorption. It is concluded that 24,25-DHCC may be a calcium-regulating hormone in man. In view of the effects demonstrated here and its relatively high concentration in plasma and slow turnover rate, 24,25-DHCC has the properties that might be ideal for a long-acting stimulator of bone mineralisation. Further work is needed to explain why 24,25-DHCC has effects in man which are not readily seen in other species.