# Presentation: 179415

## Thiazolidinedione Hepatotoxicity: The PPAR Paradigm

- Kenneth L. Hastings, Dr.P.H., D.A.B.T.
*Office of New Drugs, *
*Center for Drug Evaluation and Research, *
*US Food and Drug Administration*

## Peroxisomes

- Ubiquitous intracellular organelles
- Numerous functions
- β-oxidation of fatty acids
- Cholesterol synthesis

## Peroxisome Proliferator-Activated Receptors

- Nuclear receptors, modulate gene expression
- Common mechanism when bound by agonist: monomers dimerized with retinoid X receptor
- PPAR-RXR binds to consensus PP response elements (PPRE)
- With other response elements, produces increase/decrease in transcription of PPRE-containing genes
- Three known PPARs: α, β/δ, and γ

## Rodent Isoforms of PPAR

| Isoform | Liver | Kidney | Intestine | Spleen | Fat | Function |
| --- | --- | --- | --- | --- | --- | --- |
| α | ++++ | ++ | ++++ | + | - | Lipid metab
Reg of inflam |
| β/δ | ++ | ++ | +++ | ++ | - | Embryo
implant |
| γ | - | +/- | ++ | +++ | ++++ | Adip diff
Reg of inflam |

- Based on in situ hybridization

- Corton *et al., *2000

## Examples of PP Drugs

- Clofibrate
- Gemfibrozil
- Valproic acid
- Dehydroepiandro-sterone

- Indomethacin
- Ibuprofen
- Troglitazone
- Rosiglitazone
- Pioglitazone

## PP Carcinogenicity

- PP as a class are not genotoxic
- PPARα agonists: potent rodent hepatocarcinogens
- Associated activities: hepatomegaly and hyperplasia, increased DNA synthesis
- Suppression of apoptosis
- Associated with induction of TNFα, a liver mitogen

## PPARα Carcinogenesis

- PP

- PPARα

- Growth regulatory genes

- Lipid metabolizing genes

- Hepatocyte Proliferation

- Peroxisome proliferation

- Tumor promotion

- Liver Cancer

- Corton *et al., *2000

## PPARα Carcinogenicity

- PPARα agonists include vitamin D3 and thyroid hormones
- Hepatic effects appear to be rodent-specific
- Guinea pigs, nonhuman primates do not demonstrate peroxisome proliferation or develop hepatic tumors when exposed to agonists
- PP do not produce hepatic hypertrophy, hyperplasia, peroxisome proliferation, or hepatic tumors in PPARα-null mice

## Therapeutic PPARγ-Agonists

- Thiazolidinediones (TDZs): oral hypoglycemic agents
- PPARγ expressed on fat cells: TDZs inhibit expression of insulin-resistance factors (e.g. FFAs)
- Net increase in insulin sensitivity (liver, muscle)
- As a class, not known to be rodent hepatocarcinogens

## Marketed TDZs

- Troglitazone: withdrawn due to severe hepatotoxiciy
- Effect not clearly predicted based on nonclinical studies
- Rosiglitazone, pioglitazone now available, no clear evidence of hepatotoxicity
- However: mechanism of troglitazone hepatoxicity unknown, relevance to other TDZs unclear

## *Due to abnormal liver function

|  | TRO | PBO  | PIO | PBO  | RSG | SU/
MET | PBO |
| --- | --- | --- | --- | --- | --- | --- | --- |
| N | 2,510 | 475 | 1,526 | 793 | 3,503 | 828 | 574 |
| ALT >3x ULRR | 48 (1.9) | 3 (0.6) | 4 (0.26) | 2 (0.25) | 6 (0.17) | 4 (0.48) | 1 (0.17) |
| ALT >10x ULRR | 17 (0.68) | 0 |  |  |  |  |  |
| Dis-continued* | 20 (0.8) | 0 |  |  |  |  |  |
| Jaundice | 2 (0.08) | 0 |  |  |  |  |  |

- Lebovitz *et al., *2002

## Rate of ALT values >3 times the upper limit of the reference range per 100 patient-years of exposure in 13 double-blind, 2 active-controlled, and 7 open-label extension studies

|  | All
Rosiglitazone 
(N = 5,006)  | Placebo 
(N = 574)  | Metformin, sulfonylurea, or insulin 
(N = 1,242) |
| --- | --- | --- | --- |
| Person-years  | 5,508  | 170  | 778  |
| Incident cases  | 16  | 1  | 5  |
| Rate/100 person-years  | 0.29  | 0.59  | 0.64  |

- Lebovitz *et al., 2002*

## PPARγ Agonists

- As a class, no obvious hepatic effects
- However, in murine models of obesity and type 2 diabetes, significant upregulation of PPARγ mRNA and receptor protein
- Activation of hepatic PPARγ-responsive genes
- Chronic exposure to troglitazone, pioglitizone, or rosiglitazone produced severe hepatic centrilobular steotosis in obese or diabetic, but not lean, mice

## PPARγ Upregulation in Liver

- Enhanced uptake and intracellular binding of non-esterified fatty acids (NEFAs)
- Increased hepatic fatty acid (FA) biosynthesis
- Decreased hepatic mitochondrial FA        β-oxidation
- Net effect: hepatic steatosis and fibrosis
- Apparent inhibition of Kupffer cells

## Boelsterli and Bedoucha, 2002

## TZD-Induced Hepatic PPARγ Induction

- Hepatic microvesicular and macrovacuolar steatosis
- Kupffer cell function appears to be inhibited by PPARγ-independent pathways
- Effects observed in diseased-animal models, not healthy animals

## Boelsterli and Bedoucha, 2002

## Troglitazone Metabolism

- Unlike other TDZs, troglitazone undergoes extensive hepatic metabolism
- Sulfation, glucuronidation, and oxidation (to form quinone metabolite)
- Major metabolite in humans: sulfate
- Predominantly excreted in feces (85%)
- Troglitazone is a potent inducer of CYP3A4

*Chemical structures of troglitazone, its metabolites, pioglitazone, and rosiglitazone.*

- Nozawa *et al., *2004

## Metabolism and Hepatotoxicity

- M1 metabolite excreted in feces, undergoes extensive deconjugation and reabsorption
- Inhibits organic anion transporting polypeptide (OATP) transporters
- May inhibit transport of bile acids, bilirubin, and other anionic compounds
- Problem not seen with other TDZs

## Conclusion

- PPARs present complex toxicologic issues
- Troglitazone hepatotoxicity: not fully understood
- Diseased animal models might be useful in mechanistic studies
- TDZs that induce CYP3A4 may represent unique hazard

## References

- Boelsterli UA, Bedoucha M.  2002.  Toxicological consequences of altered peroxisome proliferator-activated receptor γ (PPARγ) expression in the liver: insights from models of obesity and type 2 diabetes.  *Biochem. Pharmacol. ***63**:1-10.
- Corton JC, Lapinskas PJ, Gonzalez FJ.  2000. Central role of PPARα in the mechanism of action of hepatocarcinogenic peroxisome proliferators. *Mutat. Res. ***448**:139-151.
- Lebovitz HE, Kreider M, Freed MI.  2002.  Evaluation of liver function in type 2 diabetic patients during clinical trials.  *Diabetes Care ***25**:815-821.
- Nozawa T, Sugiura S, Nakajima M, Goto A, Yokoi T, Nezu J-I, Tsuji A, Tamai I.  2004.  Involvement of organic anion transporting polypeptides in the transport of troglitazone sulfate: implications for understanding troglitazone hepatotoxicity.  *Drug Metab. Disp. ***32**:291-294.