Effect of the peroxisome proliferator LY171883 on hepatocellular replication in female B6C3F1 mice.

LY171883 was shown to increase the incidence of hepatocellular carcinomas and other proliferative lesions in female B6C3F1 mice. This appeared to be unrelated to the induction of peroxisomal beta-oxidation. Experiments were conducted to determine the effect of dietary LY171883 for 7 or 94 days on hepatocellular replication using continuous 7-day infusion of bromodeoxyuridine. LY171883 caused a dose-related increase in hepatocyte replication during the first 7 days, with statistical significance in the two higher dose groups. There was no effect on hepatocyte replication after 94 days of treatment. Liver weight and peroxisomal beta-oxidation were increased in the two higher dose groups after 7 and 94 days, indicating there was not a general loss of hepatic responsiveness to LY171883. The data indicate that the hepatocarcinogenesis of LY171883 in female B6C3F1 mice is not associated with sustained replication in the general population of hepatocytes. It is possible that a mitogenic effect of LY171883 exerted on spontaneously initiated cells is involved in the development of the proliferative lesions; however, further work is needed to determine this.

Hepatocellular DNA synthesis in rats given peroxisome proliferating agents: comparison of WY-14,643 to clofibric acid, nafenopin and LY171883.

The mitogenic effects of peroxisome proliferating agents have been implicated in their carcinogenicity. WY-14,643 stimulates an increase in hepatocellular DNA replication that persists with continued administration, but it is unclear if other peroxisome proliferators share this property. In these studies, WY-14,643 was compared to clofibric acid, nafenopin and LY171883 given to rats in the diet for up to 30 days. DNA replication in the rat liver was quantified by immunohistochemical methods after continuous s.c. infusion of bromodeoxyuridine by osmotic minipump. During the first 7 days of treatment, WY-14,643 (0.1% in diet) and nafenopin (0.05%) increased the percentage of bromodeoxyuridine-labeled hepatocytes to greater than 50%, from 3% in controls. Clofibric acid (0.5%) and LY171883 (0.3%) increased the labeling to approximately 33%. The replicative response to each of the compounds was localized primarily to the periportal region of the liver lobule. The time-course of replication induced by clofibric acid and WY-14,64.3 was examined over 3 day intervals. The peak of replication in response to clofibric acid occurred during days 4-6, whereas the effect of WY-14,643 peaked during days 1-3 and was much greater than clofibric acid. The replicative response to WY-14,643 persisted through 30 days at dietary concentrations of 0.1 and 0.005%. Nafenopin, LY171883 and clofibric acid were without effect on DNA replication on days 28-30 even though the hepatomegaly and induction of peroxisomal beta-oxidation persisted. Thus, under the conditions of these experiments, the persistent replicative effect through 30 days was unique to WY-14,643. Although sustained replication in the general population of hepatocytes may be involved in the carcinogenesis of WY-14,643, it does not appear to be a factor in the hepatocarcinogenesis of the other peroxisome proliferators.

Selective inhibition of cholesterol synthesis in liver versus extrahepatic tissues by HMG-CoA reductase inhibitors.

Hepatic specificity of inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase may be achieved by efficient first-pass liver extraction resulting in low circulating drug levels, as with lovastatin, or by lower cellular uptake in peripheral tissues, seen with pravastatin. BMY-21950 and its lactone form BMY-22089, new synthetic inhibitors of HMG-CoA reductase, were compared with the major reference agent lovastatin and with the synthetic inhibitor fluindostatin in several in vitro and in vivo models of potency and tissue selectivity. The kinetic mechanism and the potency of BMY-21950 as a competitive inhibitor of isolated HMG-CoA reductase were comparable to the reference agents. The inhibitory potency (cholesterol synthesis assayed by 3H2O or [14C]acetate incorporation) of BMY-21950 in rat hepatocytes (IC50 = 21 nM) and dog liver slices (IC50 = 23 nM) equalled or exceeded the potencies of the reference agents. Hepatic cholesterol synthesis in vivo in rats was effectively inhibited by BMY-21950 and its lactone form BMY-22089 (ED50 = 0.1 mg/kg p.o.), but oral doses (20 mg/kg) that suppressed liver synthesis by 83-95% inhibited sterol synthesis by only 17-24% in the ileum. In contrast, equivalent doses of lovastatin markedly inhibited cholesterol synthesis in both organs. In tissue slices from rat ileum, cell dispersions from testes, adrenal, and spleen, and in bovine ocular lens epithelial cells, BMY-21950 inhibited sterol synthesis weakly in vitro with IC50 values 76- and 188-times higher than in hepatocytes; similar effects were seen for BMY-22089. However, the IC50 ratios (tissue/hepatocyte) for lovastatin and fluindostatin were near unity in these models. Thus, BMY-21950 and BMY-22089 are the first potent synthetic HMG-CoA reductase inhibitors that possess a very high degree of liver selectivity based upon differential inhibition sensitivities in tissues. This cellular uptake-based property of hepatic specificity of BMY-21950 and BMY-22089, also manifest in pravastatin, is biochemically distinct from the pharmacodynamic-based disposition of lovastatin, which along with fluindostatin exhibited potent inhibition in all tissues that were exposed to it.

Comparison of 5-bromo-2-deoxyuridine and [3H]thymidine for studies of hepatocellular proliferation in rodents.

Hepatocyte replication traditionally has been studied by [3H]thymidine (TdR) incorporation into DNA, and more recently using incorporation of 5-bromo-2-deoxyuridine (BUdR), a synthetic analog of thymidine which is measured by immunohistochemistry. In studies to compare TdR and BUdR, mice were given the peroxisome proliferator [4-chloro-6-(2,3-xylidino)-2-pyrimidinylthio]acetic acid (WY-14643) in the diet (0.1%) for 5 days and either TdR (1 microCi/g) or BUdR (100 mg/kg) on days 2-5. The labeling index (LI) for hepatocytes of WY-14643-treated mice was 4.7% with BUdR and 5.2% with TdR. The LI for control mice was 0.3-0.4% with either label. Partially hepatectomized rats given TdR had a mean LI of 30.0 versus 32.0% in rats labeled with BUdR. Sham-operated controls given TdR had a mean LI of 0.2% and BUdR controls had a mean LI of 0.1%. Hepatectomized rats given TdR and BUdR simultaneously had an LI of 20.1% for TdR and 22.4% for BUdR. In these rats, 94.1% of the labeled cells contained both markers, whereas 1.9% had only TdR and 4.0% had only BUdR. Comparable labeling indices using either TdR or BUdR indicate that the analogs may be used interchangeably in short-term in vivo studies of liver cell proliferation.