Subchronic Urinary Bladder Effects of Muraglitazar in Male Rats1

Pioglitazone, Bladder Cancer and the Presumption of Innocence

BACKGROUND

Thiazolidinediones are potent exogenous agonists of PPAR-γ, which augment the effects of insulin to its cellular targets and mainly at the level of adipose tissue. Pioglitazone, the main thiazolidinedione in clinical practice, has shown cardiovascular and renal benefits in patients with type 2 diabetes, durable reduction of glycated hemoglobulin levels, important improvements of several components of the metabolic syndrome and beneficial effects of non-alcoholic fatty liver disease.

OBJECTIVE

Despite all of its established advantages, the controversy for an increased risk of developing bladder cancer, combined with the advent of newer drug classes that achieved major cardiorenal effects have significantly limited its use spreading a persistent shadow of doubt for its future role.

METHODS

Pubmed, Google and Scope databases have been thoroughly searched and relevant studies were selected.

RESULTS

This paper explores thoroughly both in vitro and in vivo (animal models and humans) studies that investigated the possible association of pioglitazone with bladder cancer.

CONCLUSION

Currently the association of pioglitazone with bladder cancer cannot be based on solid evidence. This evidence cannot justify its low clinical administration, especially in the present era of individualised treatment strategies. Definite clarification of this issue is imperative and urgently anticipated from future high quality and rigorous pharmacoepidemiologic research, keeping in mind its unique mechanism of action and its significant pleiotropic effects.

Human Health Effects of Biphenyl: Key Findings and Scientific Issues

BACKGROUND

In support of the Integrated Risk Information System (IRIS), the U.S. Environmental Protection Agency (EPA) has evaluated the human health hazards of biphenyl exposure.

OBJECTIVES

We review key findings and scientific issues regarding expected human health effects of biphenyl.

METHODS

Scientific literature from 1926 through September 2012 was critically evaluated to identify potential human health hazards associated with biphenyl exposure. Key issues related to the carcinogenicity and noncancer health hazards of biphenyl were examined based on evidence from experimental animal bioassays and mechanistic studies.

DISCUSSION

Systematic consideration of experimental animal studies of oral biphenyl exposure took into account the variety of study designs (e.g., study sizes, exposure levels, and exposure durations) to reconcile differing reported results. The available mechanistic and toxicokinetic evidence supports the hypothesis that male rat urinary bladder tumors arise through urinary bladder calculi formation but is insufficient to hypothesize a mode of action for liver tumors in female mice. Biphenyl and its metabolites may induce genetic damage, but a role for genotoxicity in biphenyl-induced carcinogenicity has not been established.

CONCLUSIONS

The available health effects data for biphenyl provides suggestive evidence for carcinogenicity in humans, based on increased incidences of male rat urinary bladder tumors at high exposure levels and on female mouse liver tumors. Kidney toxicity is also a potential human health hazard of biphenyl exposure.

CITATION

Li Z, Hogan KA, Cai C, Rieth S. 2016. Human health effects of biphenyl: key findings and scientific issues. Environ Health Perspect 124:703-712; http://dx.doi.org/10.1289/ehp.1509730.

Examining the safety of PPAR agonists - current trends and future prospects

INTRODUCTION

The peroxisome proliferator-activated receptor (PPAR)-α and -γ agonists, fibrates and glitazones, are effective treatments for dyslipidemia and type 2 diabetes mellitus, respectively, but exhibit class-related, as well as compound-specific safety characteristics.

AREAS COVERED

This article reviews the profiles of PPAR-α, PPAR-γ, and dual PPAR-α/γ agonists with regard to class-related and compound-specific efficacy and adverse effects. We explore how learnings from first-generation drugs are being applied to develop safer PPAR-targeted therapies.

EXPERT OPINION

The finding that rosiglitazone may increase risk for cardiovascular events has led to regulatory guidelines requiring demonstration of cardiovascular safety in appropriate outcome trials for new type 2 diabetes mellitus drugs. The emerging data on the possibly increased risk of bladder cancer with pioglitazone may prompt the need for post-approval safety studies for new drugs. Since PPAR-α and -γ affect key cardiometabolic risk factors (diabetic dyslipidemia, insulin resistance, hyperglycemia, and inflammation) in a complementary fashion, combining their benefits has emerged as a particularly attractive option. New PPAR-targeted therapies that balance the relative potency and/or activity toward PPAR-α and -γ have shown promise in retaining efficacy while reducing potential side effects.

Peroxisome proliferator-activated receptor agonists and bladder cancer: lessons from animal studies

This article reviews available animal studies on the possible link between the use of peroxisome proliferator-activated receptor (PPAR) agonists and bladder cancer, with further discussion on the possible implications to humans. Carcinogenicity studies suggest that the PPARγ agonist pioglitazone and dual PPARα/γ agonists such as ragaglitazar, muraglitazar, and naveglitazar may increase the risk of bladder cancer in a dose-responsive pattern in rats. It is interesting that bladder cancer related to PPAR agonists shows remarkable species- and sex-specificity and has a predilection to occur in the ventral dome of bladder in rodents. While male rats treated with pioglitazone or muraglitazar have a higher propensity to develop bladder cancer than female rats, mice of both sexes do not develop bladder cancer even when exposed to very high doses. Direct genotoxicity or cytotoxicity of PPAR agonists is unlikely to be the mode of action because most of the parent compounds or their metabolites of the PPAR agonists are neither mutagenic nor genotoxic, and they are rarely excreted in the urine; but a receptor-mediated PPAR effect cannot be excluded. Some suggest a "urolithiasis hypothesis" referring to the formation of urinary solids and calculi, which subsequently causes bladder necrosis, regenerative proliferation, hypertrophy, and cancer. However, whether these animal findings could have human relevance is not yet fully understood. Some argue that the urolithiasis-induced bladder cancer might be rat-specific and would probably not be applicable to humans. An effect of increased urinary growth factors induced by PPAR agonists has also been proposed, but this requires more investigations. Before fully clarified, a balance between the risks and benefits of the use of pioglitazone, an approved oral antidiabetic agent that has recently been linked to an increased but not yet confirmed risk of bladder cancer in humans, should be justified for individual use.

Suppressive effects of acid-forming diet against the tumorigenic potential of pioglitazone hydrochloride in the urinary bladder of male rats

Pioglitazone hydrochloride (PIO), a peroxisome proliferator-activated receptor gamma (PPARγ) agonist, was administered orally for 85 weeks at 16 mg/kg/day to male rats fed either a diet containing 1.5% ammonium chloride (acid-forming diet) or a control diet to investigate the effects of urinary acidification induced by the acid-forming diet on the tumorigenic potential of PIO in the urinary bladder. The surviving animals at the end of the administration period were followed to the end of the 2-year study period without changes in the diet and were subjected to terminal necropsy on Week 104. The number of urinary microcrystals, evaluated by manual counting with light microscopy and by an objective method with a laser diffraction particle size analyzer, was increased by PIO on Weeks 12 and 25 and the increases were markedly suppressed by urinary acidification. Urinary citrate was decreased by PIO throughout the study period, but no changes were seen in urinary oxalate at any timepoint. The incidences of PIO-treated males bearing at least one of the advanced proliferative changes consisting of papillary hyperplasia, nodular hyperplasia, papilloma or carcinoma were significantly decreased from 11 of 82 males fed the control diet to 2 of 80 males fed the acid-forming diet. The acid-forming diet did not show any effects on the toxicokinetic parameters of PIO and its metabolites. Microcrystalluria appears to be involved in the development of the advanced stage proliferative lesions in bladder tumorigenesis induced by PIO in male rats.

PPAR alpha and PPAR gamma coactivation rapidly induces Egr-1 in the nuclei of the dorsal and ventral urinary bladder and kidney pelvis urothelium of rats

To facilitate studies of the rat bladder carcinogenicity of dual-acting PPAR alpha+gamma agonists, we previously identified the Egr-1 transcription factor as a candidate carcinogenicity biomarker and developed rat models based on coadministration of commercially available specific PPAR alpha and PPAR gamma agonists. Immunohistochemistry for Egr-1 with a rabbit monoclonal antibody demonstrated that male vehicle-treated rats exhibited minimal urothelial expression and specifically, no nuclear signal. In contrast, Egr-1 was induced in the nuclei of bladder, as well as kidney pelvis, urothelia within one day (2 doses) of oral dosing of rats with a combination of 8 mg/kg rosiglitazone and 200 mg/kg fenofibrate (specific PPAR gamma and PPAR alpha agonists, respectively). These findings were confirmed by Western blotting using a different Egr-1 antibody. Egr-1 was induced to similar levels in the dorsal and ventral bladder urothelium, arguing against involvement of urinary solids. Egr-1 induction sometimes occurred in a localized fashion, indicating physiological microheterogeneity in the urothelium. The rapid kinetics supported that Egr-1 induction occurred as a result of pharmacological activation of PPAR alpha and PPAR gamma, which are coexpressed at high levels in the rat urothelium. Finally, our demonstration of a nuclear localization supports that the Egr-1 induced by PPAR alpha and PPAR gamma coactivation in the rat urothelium may be biologically active.

PPARgamma in Kidney Physiology and Pathophysiology

Involvement of the nuclear receptor peroxisome proliferator-activated receptor gamma (PPARγ) in kidney physiology has been explored recently. Synthetic PPARγ ligands can ameliorate the diabetic kidney disease through different mechanisms, involving inhibition of mesangial cell growth, reduction of mesangial matrix, and cytokine production of glomerular cells as well as promoting endothelial cell survival within the kidney glomeruli. Activation of PPARγ has additional profibrotic consequences, which can contribute to wound healing in diabetic glomerulonephritis. Beside many beneficial effects, PPARγ activation, however, can lead to severe water retention, a common side effect of thiazolidinedione therapy. This unwanted effect is due to the activation of PPARγ in the mesonephric distal collecting system, where PPARγ positively regulates sodium and water resorbtion leading to the expansion of interstitial fluid volume. Recent studies indicate that PPARγ is also involved in the normal kidney development, renal lipid metabolism, and activation of the renin-angiotensin system. In this paper, we give a synopsis of the current knowledge on PPARγ functions in kidney phyisology and pathophysiology.

Rat Urinary Bladder Carcinogenesis by Dual-Acting PPARalpha + gamma Agonists

Despite clinical promise, dual-acting activators of PPARalpha and gamma (here termed PPARalpha+gamma agonists) have experienced high attrition rates in preclinical and early clinical development, due to toxicity. In some cases, discontinuation was due to carcinogenic effect in the rat urothelium, the epithelial layer lining the urinary bladder, ureters, and kidney pelvis. Chronic pharmacological activation of PPARalpha is invariably associated with cancer in rats and mice. Chronic pharmacological activation of PPARgamma can in some cases also cause cancer in rats and mice. Urothelial cells coexpress PPARalpha as well as PPARgamma, making it plausible that the urothelial carcinogenicity of PPARalpha+gamma agonists may be caused by receptor-mediated effects (exaggerated pharmacology). Based on previously published mode of action data for the PPARalpha+gamma agonist ragaglitazar, and the available literature about the role of PPARalpha and gamma in rodent carcinogenesis, we propose a mode of action hypothesis for the carcinogenic effect of PPARalpha+gamma agonists in the rat urothelium, which combines receptor-mediated and off-target cytotoxic effects. The proposed mode of action hypothesis is being explored in our laboratories, towards understanding the human relevance of the rat cancer findings, and developing rapid in vitro or short-term in vivo screening approaches to faciliate development of new dual-acting PPAR agonist compounds.

Effects of PPAR agonists on proliferation and differentiation in human urothelium

Systemic treatment of rats with peroxisome proliferator-activated receptor (PPAR) agonists (mainly of dual alpha/gamma activity) has indicated that they may invoke non-genotoxic carcinogenesis in the epithelial lining of the urinary tract (urothelium). Although there is evidence in the male rat to support an indirect effect via a crystaluria-induced urothelial damage response, there is other evidence to indicate a direct signalling effect on the urothelium and hence the full implication for using these drugs in man is unclear. Numerous reports have demonstrated that PPARs are expressed within the urothelium of different species, including man, and from an early developmental stage. We have developed methods to maintain normal human urothelial (NHU) cells in culture, where the cells retain PPAR expression and express a highly proliferative phenotype, mediated via autocrine stimulation of the epidermal growth factor (EGF) receptor. We have shown that specific activation of PPARgamma results in a programme of gene expression changes associated with late/terminal cytodifferentiation, including induction of cytokeratins CK13 and CK20, tight junction-associated claudin 3, and uroplakins UPK1a and UPK2, but this is dependent upon inhibition of the signalling cascade downstream of the EGF receptor. This indicates a subtle balance in the regulation of proliferation and differentiation in urothelium, with PPARgamma agonists promoting differentiation. Our data indicate that human urothelium is a target tissue for PPARgamma signalling, but it has yet to be determined whether dual agonists could have a modulatory effect on the proliferation/differentiation balance.

PPAR Agonists and Cardiovascular Disease in Diabetes

Peroxisome proliferators activated receptors (PPARs) are ligand-activated nuclear transcription factors that play important roles in lipid and glucose homeostasis. To the extent that PPAR agonists improve diabetic dyslipidaemia and insulin resistance, these agents have been considered to reduce cardiovascular risk. However, data from murine models suggests that PPAR agonists also have independent anti-atherosclerotic actions, including the suppression of vascular inflammation, oxidative stress, and activation of the renin angiotensin system. Many of these potentially anti-atherosclerotic effects are thought to be mediated by transrepression of nuclear factor-kB, STAT, and activator protein-1 dependent pathways. In recent clinical trials, PPARalpha agonists have been shown to be effective in the primary prevention of cardiovascular events, while their cardiovascular benefit in patients with established cardiovascular disease remains equivocal. However, the use of PPARgamma agonists, and more recently dual PPARalpha/gamma coagonists, has been associated with an excess in cardiovascular events, possibly reflecting unrecognised fluid retention with potent agonists of the PPARgamma receptor. Newer pan agonists, which retain their anti-atherosclerotic activity without weight gain, may provide one solution to this problem. However, the complex biologic effects of the PPARs may mean that only vascular targeted agents or pure transrepressors will realise the goal of preventing atherosclerotic vascular disease.