Myelopreservation with the CDK4/6 inhibitor trilaciclib in patients with small-cell lung cancer receiving first-line chemotherapy: a phase Ib/randomized phase II trial.

BACKGROUND: Chemotherapy-induced damage of hematopoietic stem and progenitor cells (HSPC) causes multi-lineage myelosuppression. Trilaciclib is an intravenous CDK4/6 inhibitor in development to proactively preserve HSPC and immune system function during chemotherapy (myelopreservation). Preclinically, trilaciclib transiently maintains HSPC in G1 arrest and protects them from chemotherapy damage, leading to faster hematopoietic recovery and enhanced antitumor immunity. PATIENTS AND METHODS: This was a phase Ib (open-label, dose-finding) and phase II (randomized, double-blind placebo-controlled) study of the safety, efficacy and PK of trilaciclib in combination with etoposide/carboplatin (E/P) therapy for treatment-naive extensive-stage small-cell lung cancer patients. Patients received trilaciclib or placebo before E/P on days 1-3 of each cycle. Select end points were prespecified to assess the effect of trilaciclib on myelosuppression and antitumor efficacy. RESULTS: A total of 122 patients were enrolled, with 19 patients in part 1 and 75 patients in part 2 receiving study drug. Improvements were seen with trilaciclib in neutrophil, RBC (red blood cell) and lymphocyte measures. Safety on trilaciclib+E/P was improved with fewer ≥G3 adverse events (AEs) in trilaciclib (50%) versus placebo (83.8%), primarily due to less hematological toxicity. No trilaciclib-related ≥G3 AEs occurred. Antitumor efficacy assessment for trilaciclib versus placebo, respectively, showed: ORR (66.7% versus 56.8%, P = 0.3831); median PFS [6.2 versus 5.0 m; hazard ratio (HR) 0.71; P = 0.1695]; and OS (10.9 versus 10.6 m; HR 0.87; P = 0.6107). CONCLUSION: Trilaciclib demonstrated an improvement in the patient's tolerability of chemotherapy as shown by myelopreservation across multiple hematopoietic lineages resulting in fewer supportive care interventions and dose reductions, improved safety profile, and no detriment to antitumor efficacy. These data demonstrate strong proof-of-concept for trilaciclib's myelopreservation benefits. CLINICAL TRAIL NUMBER: NCT02499770.

Low incidence of point mutations detected in the p53 tumor suppressor gene from chemically induced rat renal mesenchymal tumors.

Previous studies have shown renal mesenchymal tumors (RMTs) induced in rats by a single intrarenal injection of nickel subsulfide and iron are more pleomorphic and metastatically aggressive than RMTs induced by a single ip injection of methyl(methoxymethyl)nitrosamine (DMN-OMe). While both RMT types contain high levels of K-ras activation, the specific mutational spectra within codon 12 of K-ras are quite different. Nickel subsulfide and iron-induced tumors exhibited codon 12 GGT-->GTT transversions exclusively, while DMN-OMe RMTs showed a wide array of codon 12 mutations, as well as mutations within codons 61 and 63 [K. G. Higinbotham, J. M. Rice, B. A. Diwan, K. S. Kasprzak, C. D. Reed, and A. O. Perantoni, Cancer Res., 52: 4747-4751, 1992; K. G. Higinbotham, J. M. Rice, and A. O. Perantoni, Mol. Carcinog., 5: 136-139, 1992]. In an effort to further correlate carcinogen-specific molecular events in renal tumors, we investigated the p53 tumor suppressor gene in RMTs induced by these two carcinogens for the presence of point mutations. The evolutionarily conserved portion of the coding region of the gene, including part of exon 4 through exon 10, was surveyed for point mutations utilizing single-strand conformation polymorphism and chemical cleavage of mismatches analyses. None (0 of 10) of the nickel subsulfide and iron-induced RMTs and only 1 of 10 DMN-OMe-induced tumors that were evaluated contained point mutations within this portion of the p53 gene. Direct sequencing of the one single-strand conformation polymorphism and chemical cleavage of mismatches-"positive" DMN-OMe-induced RMT revealed a GCC-->GTC (Ala-->Val) transition in codon 345 within exon 10. These results suggest that the different tumorigenic phenotypes exhibited by these two RMTs are not the result of specific mutations or patterns of mutations within the portion of the p53 gene examined and that the mutated p53 tumorigenic pathway, whereby p53 plays a major role in many human neoplasms, does not function in RMTs induced by either agent.

Retinoic acid activates p53 in human embryonal carcinoma through retinoid receptor-dependent stimulation of p53 transactivation function.

Although retinoids are known to regulate gene transcription by activating retinoid receptors, the targets of retinoid receptors are largely unknown. This study indicates effective all-trans retinoic acid (RA)-induced differentiation of human embryonal carcinoma cells engages p53. Unexpectedly, RA has been found to activate the transactivation function of p53 in the human embryonal carcinoma cell line, NT2/D1, in a retinoid receptor-dependent manner. A derived RA-resistant line, NT2/D1-R1, is deficient in this activity and is co-resistant to cisplatin. This indicates that RA and cisplatin responses may share a common pathway involving p53 in embryonal carcinomas. RA has no effect on p53 steady-state protein levels in either line. RA enhances endogenous p53 transactivation activity in NT2/D1 but not NT2/D1-R1 cells. In addition, RA induces transactivation activity of a gal4-p53 fusion protein, suggesting that RA activates p53 independent of increasing p53 levels or sequence-specific DNA binding. This activity is absent in retinoic acid receptor gamma (RARgamma)-deficient NT2/D1-R1 cells but can be restored upon co-transfection with specific RARs. Transient transfection of a dominant-negative p53 construct in NT2/D1 cells blocks the RA-mediated transcriptional decline of a differentiation-sensitive reporter plasmid and enhances survival of NT2/D1 cells following cisplatin treatment. Taken together, these findings indicate that RA activates the intrinsic activation function of p53 by a novel mechanism independent of effects on p53 stability or DNA binding and that this activation may be a general mechanism that contributes to RA-mediated G1 arrest.

The future beyond platinum for the treatment of advanced non-small cell lung cancer.

The development of chemotherapy for advanced non-small cell lung cancer has been most productive in its fifth decade. We began this decade by establishing cisplatin-based combination chemotherapy regimens of the 1980s as effective at improving survival for patients with advanced disease. The observed improvement in survival from these trials appears to be primarily attributed to cisplatin. Furthermore, this decade, unlike the prior, has identified an abundance of novel active agents for the treatment of this disease. Vinorelbine, gemcitabine, docetaxel, paclitaxel, and irinotecan are all new chemotherapeutic agents which have shown promising activity and their cisplatin combinations have further advanced survival for these patients. In contrast to the cisplatin-based chemotherapy trials of the 1980s, these newer chemotherapeutic agents, when given in combination with cisplatin, add to the survival outcomes for patients with advanced non-small cell lung cancer. With these survival advances has come a focus on chemotherapy-induced adverse events, lung cancer symptom management, and overall quality of life. Is it feasible to use these novel chemotherapeutic drugs as single-agents, sequentially or as nonplatinum combinations to prolong survival, minimize adverse events, control symptoms, and improve the quality of life for patients with this disease? The goal of this symposium will be to present the results of the single-agent and nonplatinum combination studies of these newer therapies with a focus on survival, symptom management, and quality of life. This symposium is intended to introduce the next decade of care for advanced non-small cell lung cancer, namely "Non-platinum-Based Chemotherapy."

The chemopreventive agent diallyl sulfide. A structurally atypical phenobarbital-type inducer.

Diallyl sulfide (DAS), a known chemopreventive agent, was administered i.g. (200 or 500 mg/kg body wt/day) to male F344/NCr rats for 4 days. Livers were removed, and hepatic levels of a variety of drug-metabolizing enzymes were determined with either catalytic assays or by quantifying levels of total cellular RNA coding for the individual genes of interest. The high dose of DAS induced the cytochrome P450 (CYP) 2B subfamily to near maximal levels [i.e. similar to those induced by phenobarbital (PB)] and induced the CYP3A subfamily, while having minimal effects on the levels of the CYP1A subfamily. In addition, DAS induced the glutathione S-transferase alpha subfamily, the glutathione S-transferase mu subfamily, and epoxide hydrolase. Unlike PB, however, DAS was also able to induce quinone oxidoreductase. In fact, the pleiotropic hepatic response to DAS appeared to be similar to that elicited by PB, with the exception that only DAS induced quinone oxidoreductase. Finally, we determined that DAS induced the levels of a specific nuclear binding protein that appears to be associated with the induction of various genes that are part of the pleiotropic response caused by PB-type inducers.

Regulation of gene expression of various phase I and phase II drug-metabolizing enzymes by tamoxifen in rat liver.

The objective of the present investigation was to evaluate the effect of tamoxifen (TAM) on the gene expression of different phase I and phase II drug-metabolizing enzymes. Groups of male and female F344/NCr rats were administered either corn oil or TAM (2.8 to 45 mg/kg body wt x 14 days) dissolved in corn oil by gavage. An additional group of rats received a diet supplemented with phenobarbital (PB, 500 ppm). Northern blot analyses of total liver RNA were conducted using [32P]-labeled cDNA or oligonucleotide probes coding for different sulfotransferase (ST); UDP-glucuronosyltransferase (UGT), glutathione S-transferase (GST), epoxide hydrolase (EPH) or cytochrome P450 (CYP) mRNA transcripts. In male rats, TAM increased the levels of STel, STa and STpl mRNAs, whereas PB increased only the STel mRNA. In female rats, there was no expression of STel and STHA mRNA in either control or TAM-treated animals. TAM and PB increased UGTBe/p mRNAs in all rats, whereas UGTml mRNA was elevated only in PB-treated animals. EPH mRNA was elevated markedly in all rats treated with TAM and PB, whereas GSTya/ye mRNA was highly increased by PB, but only marginally increased by TAM. Finally, TAM increased CYP3A1 mRNA, and slightly increased CYP2B1 mRNA, whereas PB highly elevated mRNAs for both of these CYP genes. In conclusion, treatments of rats with TAM increased the mRNA levels of many phase I and phase II drug-metabolizing enzymes, and this pleiotypic response to TAM seems to be different from other prototype inducers such as PB or dioxin (TCDD).

A markedly diminished pleiotropic response to phenobarbital and structurally-related xenobiotics in Zucker rats in comparison with F344/NCr or DA rats.

Phenobarbital (PB) and certain structurally-related compounds induce a variety of hepatic drug-metabolizing enzymes in many strains of rats. Thus, following administration of PB (300, 500 ppm), barbital (BB, 1500 ppm) or 5-ethyl-5-phenylhydantoin (EPH, 500 ppm), CYP2B1-mediated benzyloxyresorufin O-dealkylase activity and epoxide hydrolase activity were profoundly induced in female DA and F344/NCr rats. In contrast, outbred female lean and obese Zucker rats showed markedly reduced CYP2B1 responses (less than 15% and less than 5% of those observed in the female DA or F344/NCr rat) to PB (doses less than or equal to 300 ppm), BB (1500 ppm) or EPH (500 ppm). In parallel studies, profound increases in RNA levels coding for CYP2B1, glutathione S-transferases Ya/Yc (alpha subclass), or epoxide hydrolase were detected in the female F344/NCr rat following treatment with PB (300 ppm), BB (1500 ppm) or EPH (500 ppm). In contrast, lean Zucker rats showed a strong response only to the highest dose of PB (500 ppm), implying that the diminished response in the Zucker rats may occur at some pretranslational level. Similar studies with lower doses of PB, EPH or BB in male lean Zucker rats showed a decreased response, relative to that in male F344/NCr rats. However, this insensitivity was not as profound as that observed in the female Zucker rats. In fact, the response to PB-type inducers in male or female Zucker rats is probably most clearly explained as a shift of the dose-response curve sharply to the right (decreased responsiveness, compared to F344/NCr or DA rats of the same sex). This decreased responsiveness of female lean Zucker rats to induction of CYP2B1, relative to that of F344/NCr rats, was also observed with the structurally-diverse PB-type inducers clonazepam, clotrimazole and 2-hexanone. In contrast, the female Zucker rat (obese or lean) displayed a pronounced response to induction of CYP1A-mediated ethoxyresorufin O-deethylase activity by beta-naphthoflavone, a prototype inducer of CYP1A1 and CYP1A2. The Zucker rat would thus appear to represent a potentially exploitable genetic model for examining the mechanism of enzyme induction by the myriad xenobiotics which induce a PB-type response.

A pleiotropic response to phenobarbital-type enzyme inducers in the F344/NCr rat. Effects of chemicals of varied structure.

The effects of a number of phenobarbital-type inducers on selected drug-metabolizing enzymes in male F344/NCr rats were determined by measuring specific catalytic activities and/or by measuring the levels of RNA which hybridize with specific probes for the corresponding genes. The effects on hepatic CYP2B1 were assessed by measuring the levels of CYP2B1-specific RNA and benzyloxyresorufin O-dealkylase and testosterone 16 beta-hydroxylase activities. Levels of CYP3A were monitored by measuring the rate of hydroxylation of testosterone at the 6 beta-position. Microsomal epoxide hydrolase activity was determined by measurement of cellular RNA specific for this form and by assaying the hydrolysis of benzo[a]pyrene-4,5-oxide. UDP-glucuronyltransferase activity was assayed by measuring the glucuronidation of 3-hydroxybenz[a]anthracene. Levels of glutathione S-transferase Ya/Yc were measured by quantifying total cellular RNA coding for the proteins. When male F344/NCr rats were administered various doses of phenobarbital or dichlorodiphenyltrichloroethane (DDT), strong correlations between the induction of CYP2B1 and the induction of epoxide hydrolase or UDP-glucuronyltransferase activities were observed. Treatment of rats with barbiturates, hydantoins, halogenated pesticides such as DDT or alpha-hexachlorocyclohexane, 2,4,5,2',4',5'-hexachlorobiphenyl, CYP2B1 inhibitors such as clotrimazole or clonazepam, or such structurally-diverse compounds as 2-hexanone or diallyl sulfide resulted in induction of CYP2B1-mediated enzyme activity and induction of certain other forms of cytochrome P450, microsomal epoxide hydrolase, at least one form of UDP-glucuronyltransferase, and multiple forms of glutathione S-transferase. This suggests that, as a class, compounds which induce CYP2B1 also induce a coordinate hepatic pleiotropic response which includes induction of these other phase I and phase II drug-metabolizing enzymes.

Cyclin proteolysis as a retinoid cancer prevention mechanism.

The retinoids, natural and synthetic derivatives of vitamin A, are active in cancer therapy and prevention. Their biological effects are mediated through ligand-dependent interactions with retinoid receptors that associate with specific co-regulators. A better understanding of retinoid chemopreventive mechanisms is needed. Our prior work revealed that all-trans-retinoic acid (RA) prevented tobacco-specific carcinogenic transformation of cultured human bronchial epithelial cells. RA signaled G1 arrest that permitted repair of genomic DNA damage caused by these carcinogens. RA triggered G1 arrest at least partly through proteasome-dependent degradation of cyclin D1. Proteasomal inhibitors blocked RA-mediated cyclin D1 degradation. To confirm that a specific proteolysis pathway was induced by RA-treatment, a degradation assay was established using in vitro translated cyclin D1 and cellular extracts from RA-treated or untreated human bronchial epithelial cells. Incubation of RA-treated but not the control cellular extracts with in vitro translated cyclin D1 led to cyclin degradation. This degradation depended on the PEST domain of cyclin D1, implicating ubiquitination in this retinoid degradation. Retinoid receptor selective agonists demonstrated that retinoic acid receptor (RAR)beta and retinoid X receptor (RXR) but not RARalpha- or RARgamma-dependent pathways signaled this cyclin degradation. Findings were extended to the NT2/D1 human embryonal carcinoma differentiation model where a similar pathway was activated by RA-treatment. To determine whether G1 cyclins were involved directly in bronchial preneoplasia, immunohistochemical expression profiles for cyclins D1 and E were examined. Aberrant expression of these cyclins was frequent in bronchial preneoplasia. Taken together, these findings indicate that ubiquitin-dependent proteolysis of G1 cyclins is a retinoid chemoprevention mechanism. Whether the retinoids represent the optimal agents to activate this pathway is the subject of ongoing work. These findings provide a rationale for combining the retinoids in chemoprevention trials with other agents that do not activate this proteolysis pathway. What is now known about the retinoids as cancer prevention agents will be reviewed. Emphasis is placed on retinoid effects on cell cycle progression at G1.

Autofluorescence characteristics of immortalized and carcinogen-transformed human bronchial epithelial cells.

Tissue autofluorescence has been explored as a potential method of noninvasive pre-neoplasia (pre-malignancy) detection in the lung. Here, we report the first studies of intrinsic cellular autofluorescence from SV40 immortalized and distinct tobacco-carcinogen-transformed (malignant) human bronchial epithelial cells. These cell lines are useful models for studies seeking to distinguish between normal and pre-neoplastic human bronchial epithelial cells. The cells were characterized via spectrofluorimetry and confocal fluorescence microscopy. Spectrofluorimetry revealed that tryptophan was the dominant fluorophore. No change in tryptophan emission intensity was observed between immortalized and carcinogen-transformed cells. Confocal autofluorescence microscopy was performed using a highly sensitive, spectrometer-coupled instrument capable of limiting emission detection to specific wavelength ranges. These studies revealed two additional endogenous fluorophores, whose excitation and emission characteristics were consistent with nicotinamide adenine dinucleotide (NADH) and flavins. In immortalized human bronchial epithelial cells, the fluorescence of these species was localized to cytoplasmic granules. In contrast, the carcinogen-transformed cells showed an appreciable decrease in the fluorescence intensity of both NADH and flavins and the punctate, spatial localization of the autofluorescence was lost. The observed autofluorescence decrease was potentially the result of changes in the redox state of the fluorophores. The random cytoplasmic fluorescence pattern found in carcinogen-transformed cells may be attributed to changes in the mitochondrial morphology. The implications of these results to pre-neoplasia detection in the lung are discussed.

Chemoprevention of chemically-induced mammary carcinogenesis by indole-3-carbinol.

Indole-3-carbinol, a component of cruciferous vegetables, was evaluated for it efficacy in the prevention of chemically-induced mammary tumors using three different protocols. Because this compound was unstable, it was administered by gavage rather than in the diet. A preliminary dose range study revealed that dose levels of 100 and 50 mg/day, 5x/week, were not toxic to female Sprague-Dawley rats. Initial studies in the DMBA model showed that administering indole-3-carbinol during the initiation and promotion phases were highly effective chemopreventive methods (91-96% reduction in cancer multiplicity). Subsequent studies showed that the administration of indole-3-carbinol only during the initiation phase (7 days prior to until 7 days post DMBA) was also highly effective as a chemopreventive agent. Determination of enzyme levels in the livers of animals treated long-term with indole-3-carbinol showed high levels of induction of various phase I and phase II drug metabolizing enzymes. Finally, indole-3-carbinol when administered both prior to and after MNU (a direct acting carcinogen) caused a significant decrease (65%) in mammary tumor multiplicity. These results support previous studies that indole-3-carbinol can prevent mammary carcinogenesis by direct and indirect acting carcinogens. Therefore, indole-3-carbinol might be a good candidate for chemoprevention of breast cancer in women.

Effect of dietary Aroclor 1254 exposure on lung and kidney cytochromes P450 in female rats: evidence for P4501A2 expression in kidney.

In this report, we have investigated the effect of dietary exposure to Aroclor 1254 (1-100 ppm) given chronically or discontinuously over an 84-day time interval to the female F344 rat. Cytochrome P4501A was quantified in lung and kidney by measuring the dealkylation of ethoxyresorufin substrate and by Western immunoblotting. P4501A displayed a dose- and time-dependent increase in both extrahepatic organs. The kidney appeared to be more responsive to induction than lung at all doses (maximum of 500-fold induction following 84 days exposure to 100 ppm). Further, there was evidence by enzymatic activity, immunoblotting and Northern analysis of total RNA for the presence of 1A2 in the most highly induced kidneys. The decline in 1A induction observed following discontinuous exposure was more prominent in the kidney than in the lung. These data demonstrate the sensitivity of kidney to P4501A induction capacity as compared to lung, although the persistence of the induction response was evident in lung and not kidney.

Focal paraneoplastic limbic encephalitis presenting as orgasmic epilepsy.

PURPOSE: To report orgasmic epilepsy as a manifestation of paraneoplastic limbic encephalitis in a patient with small cell lung cancer. CASE REPORT: A 57 years-old woman presented with 2 month history of daily spells that consisted of a sudden pleasure provoking feeling described 'like an orgasm' lasting for 30 s to 1 min. She was a heavy smoker and had noted recent weight loss. Bronchial biopsy, following the finding of a right lung mass, confirmed the diagnosis of small cell lung cancer (SCLC). Spells subsided after starting carbamazepine. The lung cancer was treated with chemotherapy and chest radiation therapy resulting in a complete radiologic response. RESULTS: Brain magnetic resonance imaging (MRI) revealed left temporal lobe area of increased signal on T2 and FLAIR sequence. T1-weighted images after contrast administration demonstrated a circumscribed area of enhancement in the left anterior medial temporal lobe. Electroencephalogram (EEG) showed focal left mid-temporal sharp waves and intermittent slowing. Anti-Hu antibodies were detected in her serum supporting a diagnosis of paraneoplastic limbic encephalitis as the cause of her orgasmic epilepsy. The patient has been followed for 2 years after treatment without tumor recurrence or neurological deterioration. CONCLUSION: Orgasmic epilepsy is another mode of presentation of paraneoplastic limbic encephalitis leading to the diagnosis of an occult SCLC. EEG and MRI findings suggest that in this case the seizures originated from the left hemisphere. It is possible that early recognition and treatment of the SCLC will improve the prognosis of this neurologic entity.

The retinoids and cancer prevention mechanisms.

Carcinogenesis is a multistep process that converts normal cells into malignant cells. Once transformed, malignant cells acquire the ability to invade and metastasize, leading to clinically evident disease. During this continuum from normal to metastatic cells, carcinogenic steps can be arrested or reversed through pharmacological treatments, known as cancer chemoprevention. Chemoprevention strategies represent therapeutic interventions at early stages of carcinogenesis, before the onset of invasive cancer. Effective chemoprevention should reduce or avoid the clinical consequences of overt malignancies by treating early neoplastic lesions before development of clinically apparent signs or symptoms. Preclinical, clinical, and epidemiological data provide considerable support for cancer chemoprevention as an attractive therapeutic strategy. This clinical approach was validated in the recent tamoxifen randomized trial, demonstrating that a selective estrogen receptor modulator reduces the risk of breast cancer in women at high risk for this malignancy. Derivatives of vitamin A, the retinoids, have reported activity in treating specific premalignant lesions and reducing incidence of second primary tumors in patients with prior head and neck, lung or liver cancers. Whether the retinoids will prevent primary cancers at these sites is not yet known. Notably, a carotenoid (beta-carotene) was shown as inactive in primary prevention of lung cancers in high-risk individuals. This underscores the need for relevant in vitro models to identify pathways signaling chemopreventive effects. These models should assess the activity of candidate chemoprevention agents before the conduct of large and costly prevention trials. An improved understanding of cancer prevention mechanisms should aid in the discovery of new therapeutic targets and chemoprevention agents. Ideally, these agents should have tolerable clinical toxicities suitable for chronic administration to individuals at high risk for developing primary or second cancers. This article reviews what is now known from clinical and preclinical studies about the retinoids as cancer prevention agents.

Thymoma associated with syndrome of inappropriate antidiuretic hormone secretion and myasthenia gravis.

We describe the association of malignant thymoma with the syndrome of inappropriate antidiuretic hormone secretion and myasthenia gravis. Hyponatremia has not been reported associated with those tumors and our case should alert physicians about the potential for a life-threatening complication.

Relative potencies of induction of hepatic drug-metabolizing enzyme genes by individual PCB congeners.

The induction of a variety of drug-metabolizing enzymes by polychlorinated biphenyl (PCB) congeners that elicit a 2,3,7,8,-tetrachlorodibenzo-p-dioxin (TCDD)-type hepatic pleiotropic response, including 2,3,3',4,4'-pentachlorobiphenyl (BZ 105), 2,3',4,4',5-pentachlorobiphenyl (BZ 118), 2,3,3',4,4',5-hexachlorobiphenyl (BZ 156), and 3,3',4,4',5,5'-hexachlorobiphenyl (BZ 169) was examined. Following dietary exposure to the individual congeners for 5 days, livers were removed and catalytic assays for cytochrome P450 (CYP) isozymes 1A1 and 1A2 were performed. Additionally, total cellular RNA coding for hepatic drug-metabolizing genes (CYP 1A1, CYP 1A2, microsomal epoxide hydrolase, glutathione S-transferase [GST] Ya/Yc, and the TCDD-inducible isozyme of aldehyde dehydrogenase [ALDH] was quantified. 3-Methylcholanthrene (MC), TCDD, or BZ 156 (32 ppm) caused nearly maximal induction of the CYP 1A proteins but lower induction of the other genes. When the dose-response curves for induction of various drug-metabolizing genes (CYP 1A1 and 1A2, microsomal epoxide hydrolase, the GST Ya/Yc subfamily and ALDH) were examined, a spectrum of ED50s (half-maximal inductions) was observed. While CYP 1A2 exhibited an ED50 of 1.7 ppm, the induction of ALDH was shifted far to the right (ED50 > 11 ppm). Thus, different genes in a single tissue may display different dose-response characteristics. The potency (extent of induction of CYP 1A1 activity resulting from a given dietary dose) was BZ 169 >> BZ 156 > BZ 118 > BZ 105. In contrast, the potencies of the four congeners for CYP 1A1 induction were nearly equivalent when related to hepatic PCB burden, apparently due to the preferential accumulation in the liver of BZs 169 and 156 following low-level administration in the diet.

Induction of hepatic CYP1A in male F344/NCr rats by dietary exposure to Aroclor 1254: examination of immunochemical, RNA, catalytic, and pharmacokinetic endpoints.

Male F344/NCr rats were exposed to low dietary concentrations of Aroclor 1254 (0-33 ppm) for 7 days, following which the induction of selected hepatic drug metabolizing enzymes was monitored. CYP1A1, measured indirectly by assaying the O-dealkylation of ethoxyresorufin in 9000 g supernatants, was increased 1.5-, 3-, 8-, and 37-fold following 7 days of exposure to 1.0, 3.3, 10, and 33 ppm Aroclor, respectively. In contrast, the O-dealkylation of benzyloxyresorufin, an indirect measure of CYP2B1 activity, was increased approximately 4-fold following exposure to 33 ppm dietary Aroclor. Measurement of the non-P450-mediated activities epoxide hydrolase, DT-diaphorase, and aldehyde dehydrogenase (NADP+, benzaldehyde) revealed < 4-fold inductions following feeding of 33 ppm Aroclor. In view of the relatively high sensitivity of the CYP1A-specific catalytic endpoint as a biomarker for Aroclor exposure, alternative endpoints for detecting induction of this subfamily of P450 were also examined. The extent of in vivo CYP1A induction was assessed by measuring serum concentrations of zoxazolamine 150 min following an intraperitoneal dose of 100 mg/kg body wt. Slight decreases in serum zoxazolamine concentration were observed in rats exposed to as little as 1.0 ppm dietary Aroclor 1254, while profound decreases were seen in rats exposed to > or = to 10 ppm Aroclor. Immunodetection of CYP1A1 protein, with a monoclonal antibody directed against this cytochrome, revealed a 2.9-fold increase in rats exposed to as little as 1.0 ppm Aroclor, and approximately 10- and 44-fold increases following exposure to 3.3 and 10 ppm dietary Aroclor, respectively. Increases in total hepatocellular RNA coding for CYP1A1 and CYP1A2, quantified by hybridization to specific oligonucleotide probes, corresponded well to the increases in hepatic O-dealkylase activity for ethoxyresorufin (CYP1A1) and methoxyresorufin (CYP1A2), respectively. Thus, CYP1A induction, directly or indirectly measured with a variety of endpoints, represents a highly sensitive biomarker for exposure to relatively low doses of Aroclor 1254 in the rat.

Dose-response relationships for the induction of P450 2B by 1,4-bis[2-(3,5-dichloropyridyloxy)]benzne (TCPOBOP) in rat and cultured rat hepatocytes.

1. The dose-response relationships for hepatic CYP2B induction by 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) were examined in the male F344/NCr rat. TCPOBOP, administered for 14 days at 0-1000 ppm in the diet, caused concentration-dependent induction of hepatic CYP2B1 protein and RNA, and of CYP2B-mediated catalytic activities (benzyloxy- and pentoxyresorufin O-dealkylation, and testosterone 16 beta-hydroxylation). ED50 values for CYP2B induction were > or = 300 ppm dietary TCPOBOP. The maximal inductions observed were 66-88% of those resulting from exposure of the rats to 500 ppm dietary phenobarbital. 2. The EC50 values for hepatic CYP2B induction were 1.5-3.0 microM (based on serum TCPOBOP) and 15-20 mumol/kg liver. 3. The maximal inductions of isozymes of the CYP3A subfamily, of microsomal epoxide hydrolase, and of glutathione S-transferases Ya/Yc and Yb1/Yb2 in rats exposed to TCPOBOP were 58-74% of those resulting from exposure of the rats to 500 ppm dietary phenobarbital. 4. The ED50 value for induction of benzyloxyresorufin O-dealkylation in cultured rat hepatocytes by TCPOBOP was determined to be 0.93 microM. The maximal induction of this activity caused by TCPOBOP was 87% of the maximum increases caused by phenobarbital. 5. The results indicate that TCPOBOP is a highly effective phenobarbital-type inducer in the rat when administered in the diet for 2 weeks at 1000 ppm. When extent of induction is related to serum total xenobiotic level, TCPOBOP would appear to be at least as potent as, if not more potent than, phenobarbital in the rat.

In vivo induction and in vitro inhibition of hepatic cytochrome P450 activity by the benzodiazepine anticonvulsants clonazepam and diazepam.

The ability of the benzodiazepines, as a chemical class, to cause the induction and/or inhibition of cytochromes P450 has not been well characterized. In the present study, the induction of the cytochrome P450 2B subfamily (CYP2B) in vivo and the inhibition of CYP2B activity in vitro by selected benzodiazepines was examined in hepatic tissues derived from male F344/NCr rats. Initial studies of the in vivo induction or in vitro inhibition of benzyloxyresorufin O-dealkylation activity revealed that both clonazepam and diazepam were relatively effective in vivo inducers of CYP2B when administered in the diet at 500 ppm for 5 days and also were fairly potent inhibitors of the activity of these hemoproteins in vitro. Oxazepam, in contrast, was ineffective as an inducer or an inhibitor of this activity. Further studies were performed to characterize the subfamily selectivity of the P450 induction and inhibition displayed by clonazepam. Specifically, microsomes from rats treated with clonazepam (1000 or 1800 ppm in the diet for 5 days) were found to be highly induced with respect to catalytic activities mediated by CYP2B, including benzyloxyresorufin and pentoxyresorufin O-dealkylation or testosterone 16 beta-hydroxylation, but other CYP proteins were minimally induced. In addition to inducing the CYP2B subfamily, clonazepam also induced the RNA encoding other drug metabolizing enzymes (e.g., epoxide hydrolase and the glutathione S-transferase alpha-subfamily) that are typically induced by phenobarbital-type inducers. Finally, clonazepam proved to be a potent noncompetitive or "mixed-type" competitive inhibitor of catalytic activities mediated by CYP2B, but not by other CYP proteins (e.g. CYP2A, CYP3A) in microsomes derived from phenobarbital-pretreated rats.

Subchronic dietary exposure to Aroclor 1254 in rats: accumulation of PCBs in liver, blood, and adipose tissue and its relationship to induction of various hepatic drug-metabolizing enzymes.

Female F344/NCr rats were exposed continuously (7-84 days) or discontinuously (7 days exposure/21 days control diet or 28 days exposure/56 days control diet) to various dietary concentrations (1-100 ppm) of Aroclor 1254. There were dose- and time-dependent increases in PCB levels in liver, blood, and adipose tissue. Following removal of the rats from diet containing Aroclor 1254, there was a relatively rapid decrease in PCB levels, particularly in rats exposed to higher concentrations of Aroclor 1254. In parallel with the alterations in PCB levels observed, the rats showed striking dose- and time-dependent increases in hepatic levels of CYP1A1 and CYP1A2, as determined by various methods [RNA analysis, immunochemical detection, or measurement of the O-dealkylation of methoxyresorufin (CYP1A2) or ethoxyresorufin (CYP1A1)]. In rats removed from the Aroclor 1254 diet, catalytic activity for CYP1A1 as well as RNA levels for both CYP1A1 and CYP1A2 rapidly diminished. In contrast to the high levels of induction of CYP1A1 and CYP1A2 observed, limited induction (< 5-fold) of epoxide hydrolase, quinone oxidoreductase, and aldehyde dehydrogenase was detected, even in rats exposed to the highest concentration of Aroclor (100 ppm) for up to 84 days. Furthermore, induction of these non-CYP hepatic drug-metabolizing genes exhibited distinctly different concentration-response curves. The ratios of hepatic CYP1A1 activity to hepatic PCB burden were similar for rats exposed continuously to Aroclor in the diet for 7, 28, or 84 days, and for rats exposed discontinuously (7 days Aroclor/21 days control diet or 28 days Aroclor/56 days control diet). Thus, hepatic PCB levels alone appeared to be reasonably predictive of CYP1A1 levels under a variety of modes of exposure. When the ratio of CYP1A1 activity to adipose or blood PCB concentration was determined, similar ratios were observed for rats exposed continuously for 7, 28, or 84 days. However, lower ratios were observed for rats discontinuously exposed to Aroclor in the diet. These results have important implications with respect to: (a) employing PCB levels in various tissues to predict biological effects, and (b) determining different concentration-response curves for the various biological effects induced by PCBs.