Studies on the mechanism of simvastatin-induced thyroid hypertrophy and follicular cell adenoma in the rat

Towards a science-based testing strategy to identify maternal thyroid hormone imbalance and neurodevelopmental effects in the progeny - Part IV: the ECETOC and CLE Proposal for a Thyroid Function-Related Neurodevelopmental Toxicity Testing and Assessment Scheme (Thyroid-NDT-TAS)

Following the European Commission Endocrine Disruptor Criteria, substances shall be considered as having endocrine disrupting properties if they (a) elicit adverse effects, (b) have endocrine activity, and (c) the two are linked by an endocrine mode-of-action (MoA) unless the MoA is not relevant for humans. A comprehensive, structured approach to assess whether substances meet the Endocrine Disruptor Criteria for the thyroid modality (EDC-T) is currently unavailable. Here, the European Centre for Ecotoxicology and Toxicology of Chemicals Thyroxine Task Force and CropLife Europe propose a Thyroid Function-Related Neurodevelopmental Toxicity Testing and Assessment Scheme (Thyroid-NDT-TAS). In Tier 0, before entering the Thyroid-NDT-TAS, all available in vivo, in vitro and in silico data are submitted to weight-of-evidence (WoE) evaluations to determine whether the substance of interest poses a concern for thyroid disruption. If so, Tier 1 of the Thyroid-NDT-TAS includes an initial MoA and human relevance assessment (structured by the key events of possibly relevant adverse outcome pathways) and the generation of supportive in vitro/in silico data, if relevant. Only if Tier 1 is inconclusive, Tier 2 involves higher-tier testing to generate further thyroid- and/or neurodevelopment-related data. Tier 3 includes the final MoA and human relevance assessment and an overarching WoE evaluation to draw a conclusion on whether, or not, the substance meets the EDC-T. The Thyroid-NDT-TAS is based on the state-of-the-science, and it has been developed to minimise animal testing. To make human safety assessments more accurate, it is recommended to apply the Thyroid-NDT-TAS during future regulatory assessments.

Statins as Potential Chemoprevention or Therapeutic Agents in Cancer: a Model for Evaluating Repurposed Drugs

PURPOSE OF REVIEW

Repurposing established medicines for a new therapeutic indication potentially has important global and societal impact. The high costs and slow pace of new drug development have increased interest in more cost-effective repurposed drugs, particularly in the cancer arena. The conventional drug development pathway and evidence framework are not designed for drug repurposing and there is currently no consensus on establishing the evidence base before embarking on a large, resource intensive, potential practice changing phase III randomised controlled trial (RCT). Numerous observational studies have suggested a potential role for statins as a repurposed drug for cancer chemoprevention and therapy, and we review the strength of the cumulative evidence here.

RECENT FINDINGS

In the setting of cancer, a potential repurposed drug, like statins, typically goes through a cyclical history, with initial use for several years in another disease setting, prior to epidemiological research identifying a possible chemo-protective effect. However, further information is required, including review of RCT data in the initial disease setting with exploration of cancer outcomes. Additionally, more contemporary methods should be considered, such as Mendelian randomization and pharmaco-epidemiological research with "target" trial design emulation using electronic health records. Pre-clinical and traditional observational data potentially support the role of statins in the treatment of cancer; however, randomised trial evidence is not supportive. Evaluation of contemporary methods provides little added support for the use of statin therapy in cancer. We provide complementary evidence of alternative study designs to enable a robust critical appraisal from a number of sources of the go/no-go decision for a prospective phase III RCT of statins in the treatment of cancer.

Is Statin Use Associated With Risk of Thyroid Diseases? Results of a Retrospective Cohort Study

BACKGROUND

Given the ubiquity of statin use and prevalence of thyroid diseases, such as thyroid cancer, hyperthyroidism, and thyroiditis, understanding their association deserves further attention.

OBJECTIVE

To examine the association between statin use and thyroid cancer, thyrotoxicosis, goiter, and thyroiditis.

METHODS

Using Tricare data, 2 propensity score (PS)-matched cohorts of statin users and nonusers were formed: (1) a PS-matched general cohort (all patients aged 30-85 years) and (2) a PS-matched healthy cohort (excluded patients with cardiovascular diseases or severe comorbidities). Outcomes were thyroid cancer, thyrotoxicosis, goiter, and thyroiditis. Odds ratios (ORs) and 95% CIs of outcomes were estimated using conditional regression analysis.

RESULTS

Of 43 438 patients, the PS-matched general cohort matched 6342 statin users to 6342 nonusers. The OR of thyroid cancer was 0.62 (95% CI = 0.39-0.996). There was no significant difference between statin users and nonusers in risk of thyrotoxicosis (OR = 0.88; 95% CI = 0.71-1.09), goiter (OR = 0.9; 95% CI = 0.77-1.03), or thyroiditis (OR = 0.78; 95% CI = 0.53-1.15). In the PS-matched healthy cohort (3351 statin users to 3351 nonusers), there was no difference between statin users and nonusers in any outcome. Limitations of the study include its retrospective observational design and use of administrative codes in outcomes ascertainment.

CONCLUSION AND RELEVANCE

This study did not demonstrate any association of statins with harmful effects on thyroid diseases, which offers assurance to clinicians and patients. Furthermore, statin use appears to be associated with a decreased risk of thyroid cancer, but more studies are needed.

A review of species differences in the control of, and response to, chemical-induced thyroid hormone perturbations leading to thyroid cancer

This review summarises the current state of knowledge regarding the physiology and control of production of thyroid hormones, the effects of chemicals in perturbing their synthesis and release that result in thyroid cancer. It does not consider the potential neurodevelopmental consequences of low thyroid hormones. There are a number of known molecular initiating events (MIEs) that affect thyroid hormone synthesis in mammals and many chemicals are able to activate multiple MIEs simultaneously. AOP analysis of chemical-induced thyroid cancer in rodents has defined the key events that predispose to the development of rodent cancer and many of these will operate in humans under appropriate conditions, if they were exposed to high enough concentrations of the affecting chemicals. There are conditions however that, at the very least, would indicate significant quantitative differences in the sensitivity of humans to these effects, with rodents being considerably more sensitive to thyroid effects by virtue of differences in the biology, transport and control of thyroid hormones in these species as opposed to humans where turnover is appreciably lower and where serum transport of T4/T3 is different to that operating in rodents. There is heated debate around claimed qualitative differences between the rodent and human thyroid physiology, and significant reservations, both scientific and regulatory, still exist in terms of the potential neurodevelopmental consequences of low thyroid hormone levels at critical windows of time. In contrast, the situation for the chemical induction of thyroid cancer, through effects on thyroid hormone production and release, is less ambiguous with both theoretical, and actual data, showing clear dose-related thresholds for the key events predisposing to chemically induced thyroid cancer in rodents. In addition, qualitative differences in transport, and quantitative differences in half life, catabolism and turnover of thyroid hormones, exist that would not operate under normal situations in humans.

Weight of Evidence and Human Relevance Evaluation of the Benfluralin Mode of Action in Rats (Part II): Thyroid carcinogenesis

Benfluralin is an herbicide of the dinitroaniline class used to control grasses and weeds. In a 2 year dietary study in rats, benfluralin increased incidences of thyroid follicular adenoma and carcinoma at high dietary concentrations (≥2500 ppm). The benfluralin toxicology database suggests the mode of action (MOA) is initiated by induction of liver metabolizing enzymes, particularly thyroid hormone specific UGTs, a major pathway for T4 clearance in rats. As reported with phenobarbital, this effect triggers negative feedback regulation, increasing thyroid stimulating hormone (TSH) release into circulating blood. When sustained over time, this leads to thyroid changes such as follicular hypertrophy, hyperplasia and thyroid follicular tumors with chronic exposures. The described MOA was previously established in rat studies with various chemical activators of xenobiotic receptors in the liver. It is generally considered as non-relevant in humans, due to differences between humans and rats in T4 turnover and susceptibility to this carcinogenic MOA. A structured methodology based on the IPCS/MOA/Human Relevance framework was used in the evaluation of available benfluralin data, and the conclusion was determined that the carcinogenic potential of benfluralin in the thyroid is not relevant in humans.

Synergistic Anticancer Effects of Gemcitabine with Pitavastatin on Pancreatic Cancer Cell Line MIA PaCa-2 in vitro and in vivo

Background

Pancreatic ductal adenocarcinoma (PDAC) is a highly aggressive malignancy with an overall 5-year survival rate of 9.3%, and this malignancy is expected to become the second leading cause of cancer-related death by 2030. Gemcitabine resistance develops within weeks of PDAC patient’s chemotherapeutic initiation. Statins, including pitavastatin, have been indicated to have anticancer effects in numerous human cancer cell lines. Thus, in this study, we hypothesized that a combination of gemcitabine and pitavastatin may have a greater anticancer effect than gemcitabine alone on the human pancreatic carcinoma cell line MIA PaCa-2.

Methods

The anticancer effects of gemcitabine with pitavastatin were evaluated using human MIA PaCa-2 cell line in vitro and in vivo Balb/c murine xenograft tumor model. Cell viability was assessed with CCK-8, and cell migration was stained by crystal violet. Cell cycle distribution, apoptosis and mitochondrial membrane potential were examined by flow cytometry. Activation of drug transporters (hENTs, hCNTs), intracellular drug activating (dCK) and inhibition of inactivating enzymes (RRMs) pathways were assessed by Western blotting analysis. Molecular mechanisms and signaling pathways of apoptosis, necrosis and autophagy also were assessed by Western blotting.

Results

We observed that gemcitabine and pitavastatin synergistically suppressed the proliferation of MIA PaCa-2 cells through causing sub-G1 and S phase cell cycle arrest. Activation of apoptosis/necrosis was confirmed by annexin V/propidium iodide double staining, which showed increasing levels of active caspase 3, cleaved poly(ADP-ribose) polymerase and the RIP1–RIP3–MLKL complex. Moreover, gemcitabine–pitavastatin-mediated S phase arrest downregulated cyclin A2/CDK2 and upregulated p21/p27 in MIA PaCa-2 cells. Furthermore, this combination improved drug cellular metabolism pathway, mitochondria function and activated autophagy as part of the cell death mechanism. In vivo, gemcitabine-pitavastatin effectively inhibited tumor growth in a nude mouse mode of Mia PaCa-2 xenografts without observed adverse effect.

Conclusion

Combined gemcitabine–pitavastatin may be an effective novel treatment option for pancreatic cancer.

L-TRUST: Long-term risk of cancer in patients under statins therapy. A systematic review and meta-analysis

INTRODUCTION

Statins are widely prescribed drugs with established efficacy in primary and secondary prevention of cardiovascular events. Although they are mostly well tolerated, several authors have been emphasizing that the statins' safety profile is not totally clarified especially when considering risk of cancer in patients with long-term exposure to statins. This meta-analysis was aimed at evaluating the risk of cancer in patients with prolonged exposure to statins.

METHODS

Medline, Cochrane library, and clinicaltrials.gov were searched in order to identify studies with a minimum average follow-up of 10 years of exposure to statins and a cancer-related outcome reported. Relative risk (RR) of the primary outcomes and the combined effect was presented using a random-effects model. In the selected randomized control trials (RCT), statin exposure was compared with placebo, and in the selected observational studies, it was compared with no exposure to statins.

RESULTS

We retrieved 1627 studies, of which 15 full-papers were included for final review, five RCT, two cohort studies (CSs), and eight case-control studies (CCs), representing a total of 358 544 patients. Five RCT, two cohort studies (CSs), and eight case-control studies (CCs). No significant differences were found regarding risk of cancer occurrence (RR = 1.08, 0.96-1.21) or cancer mortality (RR = 0.91, 0.80-1.04) due to long-term statin exposure. Regarding all-cause mortality, a protective effect was found (RR = 0.93, 0.90-0.97).

CONCLUSIONS

According to available and published evidence, statins are not associated with an increased risk of cancer after prolonged exposure. These findings strengthen the role of statins in the primary and secondary prevention of cardiovascular events.

Image analysis for TSH mRNA in situ hybridization in pituitary glands from rats with thyroid follicular cell hypertrophy after treatment with three different test compounds

The goal of this in situ hybridization and image analysis technique is to study the effects of new pharmacological/chemical entities on the thyroid and pituitary gland in rats, reveal the pathogenesis of thyroid follicular cell hypertrophy and to retrospectively exclude the risk of thyroid tumor development in humans. In the present study, we describe the increase of thyroid-stimulating hormone- (TSH-) beta subunit mRNA in the pars distalis of the pituitary gland and the quantitative measurement of TSH mRNA positive cells from rats of three 4-week toxicity studies treated with three different test compounds inducing thyroid follicular cell and hepatocellular hypertrophy in rats. Compared to immunohistochemistry (IHC), in situ hybridization (ISH) for TSH was found to be more sensitive. With this technique we are able to exclude a direct effect of the test compound on the thyroid gland by showing the activation of thyrotrope cells from the pituitary gland and therefore this technique retrospectively enables us to exclude a possible risk for humans at an early stage of drug development. Also in case blood serum samples for evaluation of TSH are not available anymore or hepatocellular hypertrophy is not present (close metabolic relationship between thyroid gland and liver in rodents), the described method allows retrospective investigations on thyroid follicular cell hypertrophy or hyperplasia. This can be of high relevance in human safety assessment for certain drugs in order to exclude a primary effect on the thyroid gland especially when it comes to thyroid neoplasia in rodents as previously described.

The Toxicology of HMG—CoA Reductase Inhibitors: Prediction of Human Risk

The discovery that 3-hydroxy-3-methyglutaryl coenzyme A reductase was a rate-determining step in the biosynthesis of cholesterol led to the discovery of inhibitors of this enzyme. To support the development of these agents (statins) as potential hypocholesterolemic drugs, a variety of preclinical studies were conducted in several animal species. Not unexpectedly due to the central role played by mevalonic acid and its products including cholesterol in development and maintenance of cellular homeostasis, administration of high dosage levels of these agents led to the expression of a broad variety of adverse effects in many different tissues. Using the tools of toxicologic pathology and classical risk assessment, these varied toxicities were evaluated by many groups relative to the conditions of use in human therapy and a perspective was developed on potential human risk. These approaches of mechanism-based risk assessment predicted that most of the adverse effects observed in animals would not be seen under conditions of human use and supported the successful introduction of one of the most important classes of human medicines.

Two-Year Toxicity Study of Doxylamine Succinate in B6C3F1 Mice

Doxylamine succinate, a commonly used antihistamine, was administered as an admixture in the feed to groups of male and female B6C3F1 mice at dosage levels of 0,190,375, and 750 parts per million (ppm) (based on free amine) for 65 weeks (12 per group) or 2 years (48 per group). Survival to terminal sacrifice in the 2-year groups was 85–98% with no significant differences between groups of the same sex. Final body weights of the highest dose group were 3.4% and 8.7% less than controls in males and females, respectively. Doxylamine produced liver lesions in male mice including hepatocellular hypertrophy, atypical hepatocytes, clear cell and mixed cell foci, and necrosis. In females, doxylamine produced liver fatty change, hepatocellular hypertrophy, and necrosis. Doxylamine produced a significant increase in hepatocellular adenomas in the mid- and high-dosage groups of males and in the high-dosage group of females. Thyroid follicular cell hyperplasia and thyroid follicular cell adenomas also were increased in treated mice of both sexes. A treatment-related increase in cytoplasmic alteration of the parotid salivary gland in males and an increased incidence in hyperplasia of the pituitary gland in females were observed.

Statins in oncological research: from experimental studies to clinical practice

Statins, 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors are commonly used drugs in the treatment of dyslipidemias, primarily raised cholesterol. Recently, many epidemiological and preclinical studies pointed to anti-tumor properties of statins, including anti-proliferative activities, apoptosis, decreased angiogenesis and metastasis. These processes play an important role in carcinogenesis and, therefore, the role of statins in cancer disease is being seriously discussed among oncologists. Anti-neoplastic properties of statins combined with an acceptable toxicity profile in the majority of individuals support their further development as anti-tumor drugs. The mechanism of action, current preclinical studies and clinical efficacy of statins are reviewed in this paper. Moreover, promising results have been reported regarding the statins' efficacy in some cancer types, especially in esophageal and colorectal cancers, and hepatocellular carcinoma. Statins' hepatotoxicity has traditionally represented an obstacle to the prescription of this class of drugs and this issue is also discussed in this review.

The spectrum of statin therapy in cancer patients: is there a need for further investigation?

Although our understanding of the relationship between cancer and statin use continues to improve, it remains a complex association requiring further research focusing on both biologic and clinical end points in a wide range of patient populations. To date, most of the published results are from observational studies detailing the risk of incident cancers or from randomized controlled trials with cardiovascular primary end points and cancer only as a secondary end point. Although there is certainly great value in the information obtained from observational studies, they cannot prove a causal link between statins and cancer, and it would then seem appropriate to design and implement clinical trials. Such studies should consider three main end products of the mevalonate pathway (cholesterol, geranyl pyrophosphate, and farnesyl pyrophosphate) from a mechanistic perspective, as well as the potential for cancer cell mediation with statin use, in addition to pertinent clinical end points including cancer incidence and mortality.

Simvastatin and dipentyl phthalate lower ex vivo testicular testosterone production and exhibit additive effects on testicular testosterone and gene expression via distinct mechanistic pathways in the fetal rat

Sex differentiation of the male reproductive tract in mammals is driven, in part, by fetal androgen production. In utero, some phthalate esters (PEs) alter fetal Leydig cell differentiation, reducing the expression of several genes associated with steroid synthesis/transport, and consequently, lowering fetal androgen and Insl3 hormone levels. Simvastatin (SMV) is a cholesterol-lowering drug that directly inhibits HMG-CoA reductase. SMV may also disrupt steroid biosynthesis, but through a different mode of action (MOA) than the PEs. As cholesterol is a precursor of steroid hormone biosynthesis, we hypothesized that in utero exposure to SMV during the critical period of sex differentiation would lower fetal testicular testosterone (T) production without affecting genes involved in cholesterol and androgen synthesis and transport. Secondly, we hypothesized that a mixture of SMV and a PE, which may have different MOAs, would reduce testosterone levels in an additive manner. Pregnant Sprague Dawley rats were dosed orally with SMV, dipentyl phthalate (DPeP), or SMV plus DPeP from gestational days 14-18, and fetuses were evaluated on GD18. On GD18, SMV lowered fetal T production and serum triglycerides, low density lipoprotein, high density lipoprotein, and total cholesterol levels, and downregulated two genes in the fetal testis that were different from those altered by PEs. When SMV and DPeP were administered as a mixture, fetal T production was significantly reduced in an additive manner, thus demonstrating that a mixture of chemicals can induce additive effects on fetal T production even though they display different MOAs.

Statins and cancer: current and future prospects

Statins are inhibitors of 3-hydroxy-methylglutaryl (HMG) CoA reductase. They exhibit effects beyond cholesterol reduction, including anticancer activity. This review presents the effects of statins in vitro and their possible molecular anticancer mechanisms and critically discusses the data regarding the role of statins in cancer prevention. Finally, this review focuses on the use of statins combined with other chemotherapeutics to increase the effectiveness of cancer treatments. Despite rare and inconclusive clinical data, the preclinical results strongly suggest that such combined treatment could be a promising new strategy for the treatment of certain tumor types.

A clinical review of statins and cancer: helpful or harmful?

The 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins) are the second most prescribed therapeutic drug class in the United States after analgesics. Although these agents are used predominantly to reduce cholesterol concentrations in patients with hyperlipidemia, numerous studies have investigated the pleiotropic effects of statins and their potential in the prevention and/or treatment of other disease states, including cancer. Many theories have been proposed as to how statins may affect the risk or development of malignancies, prompting a clinical review of the literature. Studies have revealed statins to be associated with both increased and decreased cancer risk. Most of the published studies have been observational and retrospective in nature, and most prospective trials evaluated cancer as a secondary end point or adverse event, making it difficult to determine causality. Although most of the available evidence suggests a possible beneficial effect of statins on cancer, further study is needed with better designed trials and/or increased efforts in evaluating cancer as secondary end points in all statin trials until definite conclusions regarding statin effects on cancer risk and occurrence can be made.

Screening statins for possible carcinogenic risk: up to 9 years of follow-up of 361,859 recipients

PURPOSE

Determine the risk of cancer in statin users.

METHODS

Risk of cancer in up to 9.4 years after first recorded receipt of statins was evaluated in subscribers of an integrated health care program in northern California. Statin use and cancer development were ascertained from the program's pharmacy records and cancer registry from August 1994 to December 2003.

RESULTS

Most of the 361,859 statin users received lovastatin, simvastatin or both. Results are presented from analyses with 2-year lag and use for over 5 years. Most of the observed associations were likely due to chance or confounding. The few associations that seemed less readily explainable were increased risk of cancers of the thyroid, esophagus and urinary tract and decreased risk of colon cancer in men. Increased risk of lung cancer was the only nominally statistically significant positive association in women and could be partially attributable to their smoking habits.

CONCLUSIONS

Overall this study provided no strong evidence of either causation or prevention of cancer by statins.

The role of statins in cancer therapy

Administration of 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors, or statins, to ambulatory patients is associated with a lower incidence of long-term adverse cardiovascular events, including death, myocardial infarction, stroke, atrial fibrillation, and renal dysfunction. However, increasing clinical evidence suggests that statins, independent of their effects on serum cholesterol levels, may also play a potential role in the prevention and treatment of cancer. Specifically, statins have been shown to exert several beneficial antineoplastic properties, including decreased tumor growth, angiogenesis, and metastasis. The feasibility and efficacy of statins for the prevention and treatment of cancer is reviewed.

Preclinical Development of New Drugs that Enhance Thyroid Hormone Metabolism and Clearance: Inadequacy of Using Rats as an Animal Model for Predicting Human Risks in an IND and NDA

New drugs that enhance metabolism or clearance of thyroid hormones in rats often trigger a sequence of toxicity events during chronic administration: reduction of thyroxine, elevation of thyroid-stimulating hormone (TSH) levels, and thyroid gland hyperfunction/growth. Hepatocellular hypertrophy and thyroid follicular hyperplasia are often observed with increased liver and thyroid organ weights. This unique toxicity profile seems to be species-specific because the thyroxine in rodents is metabolized rapidly, without thyroid hormone-binding globulin that serves as a reserve, as in humans. Thus, elevations of TSH were not reported in humans for drugs such as delavirdine, fluvastatin, nicardipine, phenobarbital, simvastatin, and spironolactone, all of which produce thyroid hyperplasia or tumors in rats. Further, the human thyroid is less sensitive to prolonged TSH stimulation than that of the rat (eg, endemic goiter patients with high TSH due to iodine deficiency do not develop thyroid cancer). In view of the species difference in sensitivity of the thyroid between rodents and humans, using the rat as an animal model to explore target organs of toxicity for a new drug that significantly enhances thyroid hormone metabolism/clearance and increases TSH levels would not be adequate. In this case, a compromised and dysfunctional hypothalamo-pituitary-thyroid system would confound the toxicity profile explored in preclinical toxicity testing and render the model an inadequate risk predictor for the new drug in humans. Under such conditions, IND and NDA sponsors of drugs exhibiting this activity profile should be encouraged to use alternative animal species for toxicity exploration to provide a more meaningful human risk prediction.

Thyroid lesions and dioxin accumulation in the livers of jungle crows (Corvus macrorhynchos) in urban and suburban Tokyo

Wild jungle crows (Corvus macrorhynchos) captured from three different areas of Tokyo were examined to evaluate environmental contamination of dioxins. In addition to the pathologic examination of their whole body, accumulation of dioxins, mRNA expression of the aryl hydrocarbon receptor (AhR), and pentoxyresorufin-O-depenthylase (PROD) activity in the liver were determined. Marked histopathologic changes were observed in the thyroid glands, especially in the crows from the urban downtown area. Levels of dioxins and their toxic equivalents (TEQs) and AhR mRNA expression in the livers of the crows from the urban area were higher than those from the suburban area. There was a high correlation between the levels of TEQs and PROD activity. The results of the present study demonstrated that jungle crows possess AhR-mediated toxicologic pathways similar to those of mammals and suggest the possibility that the thyroidal changes observed in the adult crows from the urban areas are one of the toxic manifestations resulting from exposure to dioxins and other environmental chemicals.

Selenoproteins, Cholesterol-Lowering Drugs, and the Consequences Revisiting of the Mevalonate Pathway

3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins) and peroxisome proliferator-activated receptor alpha activators (fibrates) are the backbone of pharmacologic hypercholesterolemia and dyslipidemia treatment. Many of their clinical effects, however, are still enigmatic. This article describes how a side road of the mevalonate pathway, characterized in recent years, can rationalize a major fraction of these unexplained observations. This side road is the enzymatic isopentenylation of selenocysteine-tRNA([Ser]Sec) (Sec-tRNA), the singular tRNA to decode the unusual amino acid selenocysteine. The functionally indispensable isopentenylation of Sec-tRNA requires a unique intermediate from the mevalonate pathway, isopentenyl pyrophosphate, which concomitantly constitutes the central building block for cholesterol biosynthesis, and whose formation is suppressed by statins and fibrates. The resultant inhibition of Sec-tRNA isopentenylation profoundly decreases selenoprotein expression. This effect might seamlessly explain the immunosuppressive, redox, endothelial, sympatholytic, and thyroidal effects of statins and fibrates as well as their common side effects and drug interactions.

Increased levothyroxine requirements presenting as "inappropriate" TSH secretion syndrome in a patient with nephrotic syndrome

Patients with primary thyroid failure on levothyroxine (LT4) replacement who develop nephrotic syndrome (NS) may rarely present with an increase in LT4 requirements. In this report, we describe a patient with thyroid failure following radioactive iodine ablation for Graves' disease who required an escalation of LT4 doses following the onset of NS. The case presented with disproportionately elevated TSH levels in the presence of normal (or slightly subnormal) thyroid hormone levels, thus, masquerading as a state of "inappropriate" TSH secretion. This pattern of extreme dysregulation in thyroid function indices due to urinary loss of thyroid hormones has not been previously described in NS, and, therefore, extends the spectrum of endocrine manifestations of NS.

Potential molecular targets for manipulating the radiation response

Recent advances in our understanding of the molecular events that occur following ionizing radiation leading to DNA damage and repair, apoptosis, and cell-cycle arrests suggest new ways in which the radiation response might be manipulated. Specific targets which, if inactivated, might increase radiosensitivity include Ras, which has been implicated in the radioresistant phenotype, and components of DNA-dependent protein kinase or other molecules involved in the recognition or repair of DNA damage. In some tumors, apoptosis is an important mode of cell death following radiation, so agents that promote this may prove useful therapeutically. Conversely, side effects may result from radiation-induced apoptosis of normal tissues: for example, pneumonitis following the destruction of endothelial cells in the pulmonary vasculature. Therefore, decreasing apoptosis in these tissues may reduce late effects. It may also be possible to prevent late effects such as fibrosis by blocking the induction of certain genes such as transforming growth factor beta. Cell-cycle regulation is another area that could be manipulated to increase radiosensitivity. There is evidence that the G2 delay following radiation is important in protecting cells from death. Abolition of this delay may increase radiosensitivity, especially in cells with mutant p53 that have lost the G1 checkpoint.

Effect of fluvastatin, an inhibitor of 3-hydroxy-3-methyl glutaryl CoA reductase, on drug-metabolizing enzymes in rats

Fluvastatin (FV), a new cholesterol-lowering agent, has been studied for its effects on hepatic microsomal drug-metabolizing enzymes in male rats. FV was orally administered in dosages of 1, 5, and 30 mg/kg/day for 7 consecutive days. 3-Hydroxy-3-methylglutaryl CoA (HMG-CoA) reductase activities were markedly decreased at all dose levels. The amount of microsomal protein and the contents of cytochromes P450 and b5 did not change. No induction of aniline hydroxylase, aminopyrine N-demethylase, testosterone hydroxylases (15 alpha-, 7 alpha-, 6 beta-, 16 alpha-, and 16 beta-), and UDP-glucuronosyltransferase were found. On the other hand, 7-ethoxycoumarin o-deethylase activity was slightly increased and lauric acid omega-1-hydroxylase activity tended to be decreased after treatment with FV.

Comparison of properties of four inhibitors of 3-hydroxy-3-methylglutaryl-coenzyme A reductase

Four inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase have been approved for treatment of hypercholesterolemia. Three of these are fungal metabolites or derivatives thereof: lovastatin, simvastatin, and pravastatin. The fourth, fluvastatin, is totally synthetic. Its structure, containing a fluorophenyl-substituted indole ring, is distinct from that of the fungal metabolites. Lovastatin and simvastatin are administered as prodrugs, which undergo in vivo transformation to active inhibitory forms; fluvastatin and pravastatin are administered as active agents. The HMG-CoA reductase inhibitors are all effective in reducing plasma concentrations of low density lipoprotein. They have differing pharmacokinetic properties, which may be of importance in some patients. All of these drugs are very well tolerated, and there do not appear to be major differences in toxicity or adverse effects. When LDL reductions > 30% are needed, simvastatin is the most cost-effective HMG-CoA reductase inhibitor. However, these drugs are most commonly used in dosages that reduce LDL-C by 20-30%. For this degree of LDL reduction, fluvastatin is the most cost-effective HMG-CoA reductase inhibitor.

Tissue selectivity of hydroxymethylglutaryl coenzyme A (HMG CoA) reductase inhibitors

Hydroxymethylglutaryl coenzyme A (HMG CoA) reductase inhibitors are a class of lipid-lowering medications, with a major activity on plasma cholesterol levels, now enjoying a vast popularity among physicians and patients. These drugs, affecting a very early and key step of sterol biosynthesis, differ to a large extent in their physicochemical properties, tissue distribution and side effects in animals, possibly in humans. Some of these agents (namely lovastatin and simvastatin) are strikingly lipophilic and require enzymatic conversion from the lactone to the open-ring forms, whereas pravastatin, active per se, is hydrophilic. Liver uptake of pravastatin is regulated by a carrier-mediated mechanism. Other HMG CoA reductase inhibitors have been designed, with the objective of obtaining high levels of hepato-selectivity. Evaluation of available data in terms of potential advantages in tissue, namely liver selectivity, of HMG CoA reductase inhibitors, suggests, that, indeed, altered sterol biosynthesis in a number of tissues may potentially result in the appearance of significant side effects. While there is no clear-cut relationship between tissue selectivity and lipophilicity, the presence of this latter feature seems, in general, to dictate a lesser absorption to peripheral tissues vs the liver. At present, the toxicological profile of major HMG CoA reductase inhibitors appears safe; it is, however, possible that in selected patient groups liver selectivity may offer a considerable therapeutic advantage.