Topics in clinical pharmacology: ganciclovir

The effectiveness and safety of Tanreqing Injection combined with ganciclovir on the treatment of respiratory syncytial virus pneumonia in children: A protocol for systematic review and meta-analysis of randomized controlled trials

BACKGROUND

Based on the Theory of Traditional Chinese Medicine, this study systematically evaluated the effectiveness and safety of Chinese medicine preparation Tanreqing injection combined with ganciclovir on the treatment of respiratory syncytial virus pneumonia in children, and provided new ideas and methods for the treatment of respiratory syncytial virus pneumonia (RSVP) in children. At the same time, it also studies the effectiveness and safety of the combination of Chinese and Western medicine on the treatment of related diseases from the direction of evidence-based medicine.

METHODS

The relevant literature was searched by the computer in the electronic network databases, the retrieved databases include Chinese database and English database, English database includes PubMed, Cochrane Library, Embase and Web of Science. Chinese database includes: CNKI, SinoMed, WangFang Date, VIP and other networks electronic full-text database, conducting a randomized controlled trial of Tanreqing Injection combined with ganciclovir (study group) and ganciclovir alone (control group) on the treatment of RSVP in children and the retrieval time limit is set from the establishment of each database to July 1, 2020. According to the inclusion and exclusion criteria, the literature is independently searched and screened by 2 researchers, and conducting the full-text retrieval and evaluation of the research to be included, and extracting the information and checking it after reading the full-text; In case of disagreement, a third researcher will be invited to participate, and the decision is made after discussion by the 3 researchers. They were using the bias risk assessment tool provided by the Cochrane Handbook for Systematic Reviews of Interventions 3.0.2 to evaluate the selected literature. They were using RevMan 5.3 statistical software to conduct statistical analysis.

RESULTS

This study will be carried out in full accordance with the steps of systematic review as required in the Cochrane Handbook for Systematic Reviews of Interventions. All research results will be published publicly in international academic journals with peer review.

CONCLUSION

After the meta-analysis of Tanreqing injection combined with ganciclovir on the treatment of RSVP in children, this paper will give a scientific and objective judgment on the effectiveness and safety of the combined use of Chinese and Western medicine on the treatment of RSVP in children, to provide evidence-based medical evidence for the clinical application, effectiveness and safety of Chinese and Western medicine combined on the treatment of RSVP in children.

PROSPERO REGISTRATION NUMBER

OSF platform, registration number: j2bz5.

CMV-specific immune reconstitution following allogeneic stem cell transplantation

Cytomegalovirus (CMV) remains a major contributor to morbidity and mortality following allogeneic haemopoietic stem cell transplant (HSCT) despite widespread use of viraemia monitoring and pre-emptive antiviral therapy. Uncontrolled viral replication occurs primarily in the first 100 d post transplant but this high risk period can extend to many months if immune recovery is delayed. The re-establishment of a functional population of cellular effectors is essential for control of virus replication and depends on recipient and donor serostatus, the stem cell source, degree of HLA matching and post-transplant factors such as CMV antigen exposure, presence of GVHD and ongoing use of immune suppression. A number of immune monitoring assays exist but have not yet become widely accessible for routine clinical use. Vaccination, adoptive transfer of CMV specific T cells and a number of graft engineering processes are being evaluated to enhance of CMV specific immune recovery post HSCT.

Antisense approach for the treatment of cytomegalovirus infection

Human cytomegalovirus (HCMV) is the most common viral opportunistic infection in patients suffering with acquired immunodeficiency virus (AIDS). HCMV is a systemic infection that may infect several sites in the body, including the retina, gastrointestinal tract, lungs, liver, and central nervous system. Retinitis is the most frequent manifestation of HCMV infection, occurring in 15-40% of all patients. HCMV is progressive and destroys the retina, eventually leading to blindness. Although, there are several drugs available to treat this disease, they are often of limited efficacy and have significant side-effects. Antisense oligonucleotides represent a novel alternative to the currently available drugs. Due to their high affinity and specificity to target the HCMV RNAs, interest in antisense technology to treat HCMV infections has been intense during the past few years. Two antisense drugs are currently in clinical trials, ISIS 2922 (Formivirsen) and GEM 132.

Adenovirus HSV-TK construct with thyroid-specific promoter: enhancement of activity and specificity with histone deacetylase inhibitors and agents modulating the camp pathway

The successful use of tissue- or tumor-selective promoters in targeted gene therapy for cancer depends on high and selective activity. Tg is a thyroid-specific protein that is expressed in the normal thyroid and a majority of thyroid tumors. In the present study, we show, using a luciferase reporter assay, that a construct containing the putative Tg promoter and enhancer is active in 4 thyroid carcinoma cell lines (including 2 anaplastic thyroid carcinoma cell lines) and not in 5 cancer cell lines arising from nonthyroid tissues. Furthermore, both the activity and the specificity of this construct were increased by pretreatment with 8-Br-cAMP and the histone deacetylase inhibitor depsipeptide (FR901228). Expression of thymidine kinase in thyroid cancer cells infected with a recombinant adenovirus (Ad) carrying a Tg enhancer/promoter-thymidine kinase expression cassette (AdTg enhancer/promoter-TK) correlated with the level of Tg enhancer/promoter activity in these cells. Under similar conditions, TK expression was not observed in cancer cell lines arising from nonthyroid tissues. Cells infected with AdTg enhancer/promoter-TK demonstrated preferential GCV sensitivity, with up to a 100,000-fold increase in GCV sensitivity in thyroid cancer cell lines compared to cancer cell lines of nonthyroid origin. The construct described herein can be used to selectively target thyroid cancer cells, and its expression can be modulated to further increase its specificity and selectivity, especially in anaplastic thyroid carcinoma cells, using 8-Br-cAMP and depsipeptide.

Gene therapy utilizing the Cre/loxP system selectively suppresses tumor growth of disseminated carcinoembryonic antigen-producing cancer cells

Recent clinical trials of cancer gene therapy have shown encouraging results for controlling localized tumors. However, to control metastatic or disseminated tumor cells, further modification of vectors is required to enhance specificity and infectivity against targets. We investigated whether utilization of the Cre recombinase(Cre)/loxP system contributes to enhanced antitumor effects together with minimal adverse reactions in specific gene therapy against disseminated carcinoembryonic antigen (CEA)-producing cancer cells in the peritoneal cavity of mice. CEA-producing cancer would be a good therapeutic target because it is found in lung, stomach and colon sites, which account for most cancers. We constructed a pair of recombinant adenoviral vectors (Ads), one of which expresses the Cre gene under the control of the CEA promoter (Ad.CEA-Cre); the other expresses the herpes simplex virus thymidine kinase (HSV-TK) gene (Ad.lox-TK), or the beta-galactosidase gene (beta-gal) by Cre (Ad.lox-beta-gal). Intraperitoneal coinjection of Ad.CEA-Cre and Ad.lox-beta-gal into mice with peritonitis carcinomatosa by CEA-producing tumor cells showed selective expression of the beta-gal gene in tumor foci. Coinfection of Ad.CEA-Cre and Ad.lox-TK followed by ganciclovir (GCV) administration significantly suppressed the total tumor weight in the peritoneal cavity of the mice to 13% of that of the untreated mice and 22% of that of the mice treated with Ad.CEA-TK/GCV, an Ad that expressed the HSV-TK gene driven by the CEA promoter alone. Moreover, treatment with Ad.CEA-Cre and Ad.lox-TK/GCV completely suppressed tumors in 4 of 10 (40%) mice without significant weight loss, although 2 of 10 mice treated with Ad.CAG-TK/GCV, an adenovirus vector that strongly but nonspecifically expressed the TK gene, died due to severe side effects including diarrhea, weight loss and liver dysfunction. These findings suggest that cell type-specific gene therapy using the Cre/loxP system is effective against disseminated cancer cells without significant side effects.

Transient control of a virus-induced immunopathology by genetic immunosuppression

The ability to control T cell reactivity using suicide genes opens new perspectives for the treatment of T cell-mediated diseases. The therapeutic effect is achieved by the selective killing of thymidine kinase gene-modified activated T cells by ganciclovir (GCV). This strategy has been shown to control T cell alloreactivity efficiently after bone marrow or solid organ transplantation. Here, we aimed to determine whether an immunopathological process induced by a viral infection could be controlled by GCV when T cells express a thymidine kinase transgene. When transgenic mice were infected with the lymphocytic choriomeningitis virus, administration of GCV resulted in an efficient, but only transient, control of the immunopathological immune response. Further analysis revealed the existence of a minute population of GCV-insensitive T cells. These cells expand in response to the virus despite the presence of GCV and cause immunopathology before viral elimination is finally obtained. Thus, when confronted with a replicative virus, the efficacy of this genetic immunosuppression strategy is highly dependent on the presence of even small numbers of GCV-insensitive cells. These results emphasize the need for sufficient preclinical investigations with regard to the pathology and the nature of the immune response if suicide gene transfer is envisioned for new therapeutic indications.

Human cytomegalovirus: challenges, opportunities and new drug development

In the age of highly active antiretroviral therapy, the incidence of human cytomegalovirus (HCMV) retinitis in AIDS patients has decreased substantially. However, this change does not indicate that HCMV disease in AIDS patients and other immunocompromised patients has abated and is no longer a concern. On the contrary, HCMV disease in graft recipients, newborns, and even in AIDS patients still accounts for considerable morbidity, and drug resistance to the anti-HCMV compounds is a major problem. Furthermore, HCMV may have a role in metabolic diseases, such as atherosclerosis. Fortunately there are novel and potentially very effective new compounds undergoing pre-clinical and clinical evaluation. These developments point the way toward new therapies and also to a clearer understanding of the biology of HCMV replication, infection and disease.

Utilization of variant-type of human alpha-fetoprotein promoter in gene therapy targeting for hepatocellular carcinoma

We previously reported that the retroviral vector (LNAFW0.3TK) expressing the herpes simplex thymidine kinase (HSVtk) gene under the control of the 0.3 kb human alpha-fetoprotein (AFP) promoter provided the ganciclovir (GCV)-mediated cytotoxicity in the high AFP-producing (HuH-7) but not in the low AFP-producing (huH-1/cl.2) human hepatoma cells. In the present study, we constructed the retroviral vector (LNAFM0.3TK) in which the HSVtk gene expression is regulated by the variant-type of the 0.3 kb human AFP promoter with a G-to-A substitution at nucleotide -119, a point mutation responsible for hereditary persistence of human AFP and the vector was applied to three human hepatoma cell lines HuH-7, huH-1/cl.2 and intermediate AFP-producing cells (PLC/PRF/5). By the reporter gene transfection assay, the activity of the variant-type of the promoter was much higher than that of the wild-type of the promoter in both HuH-7 and huH-1/cl.2 cells. Consistent with this, LNAFM0.3TK infection could sensitize huH-1/cl.2 cells, as well as HuH-7 and PLC/PRF/5 cells to GCV, but did not affect cell growth of nonhepatoma cells (HeLa). In addition, the bystander effect was achieved more efficiently by LNAFM0.3TK infection than LNAFW0.3TK infection in HuH-7 cells. These results suggest that the variant-type of the human AFP promoter ensures the therapeutic gene expression in gene therapy particularly for the low AFP-producing hepatoma cells.

Dose and duration-dependence of ganciclovir treatment against murine cytomegalovirus infection in severe combined immunodeficient mice

The present study investigates the full dose-response curve and treatment duration dependence of ganciclovir (GCV) against murine cytomegalovirus (MCMV) infection in severe combined immunodeficiency (SCID) mice. Animals inoculated intraperitoneally with 6.3 x 10(3) pfu of MCMV per mouse developed typical wasting syndrome rapidly and died around day 12 post-inoculation. Once-daily treatment with subcutaneous GCV for 5 days dose dependently delayed MCMV-induced wasting syndrome and mortality at a dose range of 1-80 mg/kg per day, whereas a dose of 160 mg/kg per day induced reversible side-effects. The effect of GCV treatment on mean death day (MDD) was significantly correlated to reductions of viral titers in the lung (r = 0.969, P < 0.05). Treatment duration dependence was examined at the dose of GCV at 80 mg/kg per day for 1, 5, 8 and 12 days. The protective duration, over vehicle-treated mice, was constantly 3-4 days plus the duration of GCV treatment, as evidenced by the delay of viral replication, wasting syndrome and death. At a sub-optimally effective dose of 10 mg/kg per day of GCV, maximum protection was achieved with a 8-day treatment regimen. Prolongation of this treatment to 12 days failed to further delay mean death day and wasting syndrome that started on day 10, indicative of insufficient suppression of viral replication. Treatment with a single dose of GCV failed to show a complete dose-response curve since only minimal protective effects were observed at the dose of 80 mg/kg while side-effects were associated with the dose of 160 mg/kg. The treatment duration dependence and requirement for sufficient dosage of GCV against CMV infection observed in the current model are consistent with clinical observations. It also suggests that 5 8 days treatment duration may be a good balance considering the opportunity for identifying active compounds and speeding up the turnaround time in drug evaluations.

In vivo adenovirus-mediated prodrug gene therapy for carcinoembryonic antigen-producing pancreatic cancer

In gene therapy for malignancy, the herpes simplex virus thymidine kinase (HSVtk)-ganciclovir (GCV) system has been widely used. For pancreatic cancer targeting, we estimated the therapeutic efficacy of gene transduction by an adenovirus-carrying HSVtk gene under the control of a carcinoembryonic antigen (CEA) promoter (AdCEAtk) followed by systemic administration of GCV. Four cell lines, CEA-producing Su.86.86. BxPC-3 (pancreatic cancer cells), MKN45 (gastric cancer cells) and CEA-nonproducing HeLa, were used for analysis of GCV sensitivity induced by adenoviral gene transduction. To evaluate the therapeutic efficacy of AdCEAtk and GCV administration in human CEA-positive pancreatic cancer in vivo, a subcutaneously implanted tumor-bearing nude mouse model was used. When the HSVtk gene was transduced with a ubiquitous promoter into these cells, increase of the GCV sensitivity was independent of CEA-production. In contrast, when the cells were transduced with a CEA promoter, the cell-killing effect of GCV was increased in only CEA-producing cells. For in vivo analysis, AdCEAtk was delivered into subcutaneously established tumors of Su.86.86 cells. Immunohistochemical staining of the tumor showed that HSVtk protein was expressed only in tumor cells, and tumor growth was markedly suppressed by administration of GCV. These results suggest that the adenovirus-mediated transfer of HSVtk gene with CEA promoter specifically increases the GCV sensitivity of CEA-producing pancreatic cancer cells in vitro and in vivo. This strategy may provide a useful tool for treating pancreatic cancer, especially CEA-producing tumor cells.

Pharmacokinetics and clearance of ganciclovir during continuous hemodiafiltration

OBJECTIVE

To evaluate the ganciclovir pharmacokinetics and clearance during continuous venovenous hemodiafiltration.

DESIGN

Case report.

SETTING

General intensive care unit of a tertiary care emergency department.

PATIENTS

A 63-yr-old female who has a history of active behçet's disease that has been controlled with oral prednisolone, and who has chronic renal failure.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

A 5-mg/kg dosage of ganciclovir was administered intravenously over a 60-min period under continuous venovenous hemodiafiltration. Samples from the arterial and venous blood catheters and from the ultradiafiltrate were collected over the next 12 hrs to calculate pharmacokinetic parameters and clearance of hemodiafiltration. The pharmacokinetic parameters were as follows: half-life of elimination phase 12.6 hrs; total clearance 0.55 mL/min/kg; and volume distribution of steady state 27.07 L. The clearance of hemodiafiltration was 0.63 mL/min/kg.

CONCLUSION

Continuous venovenous hemodiafiltration is effective in removing ganciclovir from the blood.

Adenovirus-mediated gene therapy of gastric carcinoma using cancer-specific gene expression in vivo

The carcinoembryonic antigen (CEA) is a glycoprotein which overexpressed in the majority of human gastric cancers. We demonstrated that recombinant adenoviral vector (AdCEAtk), containing the CEA promoter, could transfer the herpes simplex virus thymidine kinase (HSVtk) gene into CEA-producing gastric cancer cells to confer sensitivity to ganciclovir (GCV) in vivo. In an ex vivo experiment, the tumor growth was inhibited after GCV treatment when the tumor contained more than 20% of AdCEAtk infected cells, indicating an efficient bystander killing effect. With intra-tumoral injection of AdCEAtk, the HSVtk were selectively expressed in approximately 30% of CEA producing cancer cells. By AdCEAtk injection and GCV administration, the growth of tumors was significantly inhibited by 20% as compared to untreated tumors. It is hoped that these results provide a strategy of tumor specific gene transfer for CEA producing gastric cancers.

Conditional ablation of cerebellar astrocytes in postnatal transgenic mice

Astrocytes have been proposed to have multiple roles in the development and maintenance of the vertebrate CNS. To facilitate documentation of these roles, we designed a transgene to enable their ablation at selectable times. The transgene consists of the coding region for the herpes simplex virus-thymidine kinase (HSV-TK) under the control of the human glial fibrillary acidic protein gene promoter. The HSV-TK is innocuous but converts the antiherpetic agent ganciclovir (GCV) to a toxic product that interferes with DNA replication in proliferating cells. In a developmental study, transgenic mice were treated with GCV during the first postnatal week, with evaluation at P19. Treated mice displayed severe ataxia. Histological examination revealed disrupted astrocyte development, particularly in the cerebellum, with marked secondary effects on other cell types. Cerebellar defects included a loss in the numbers of astrocytes and an overall reduction in cerebellar size and disruption of the normally well defined cellular layers. Radial glia were disordered, Purkinje cells were ectopically distributed and displayed abnormal dendritic trees, and granule cells were markedly depleted. These effects were more severe in animals treated on postnatal day 1 versus treatment at day 5. A major factor causing granule cell death was excitotoxicity attributable to activation of NMDA receptors. These results suggest a critical role for astrocytes in cerebellar development.

Population pharmacokinetics of ganciclovir in newborns with congenital cytomegalovirus infections. NIAID Collaborative Antiviral Study Group

The population pharmacokinetics of ganciclovir was investigated in a group of 27 newborns with symptomatic congenital cytomegalovirus infection by nonlinear mixed-effects modeling analysis. Individual characteristics including approximated creatinine clearance from serum (ASCC) and body weight (WGE) were identified to significantly influence total clearance from plasma (CL) and the apparent total volume of distribution (V) of ganciclovir, respectively. The regression equations used to model these relationships were expressed as CL (in liters per hour) = 0.262 + (0.00271 x ASCC) and V (in liters) = 0.627 + (0.437 x WGE). By using this model, typical values of the pharmacokinetic parameter CL and V were 0.428 +/- 0.079 liters/h and 1.773 +/- 0.320 liters, respectively. Upon validation with a larger number of newborns, this model should allow for the definition of possible relationships between the pharmacokinetic disposition of ganciclovir and pharmacodynamic events in neonates.

Drug interactions: how they affect people living with HIV/AIDS

Given the diversity and increasing life span of HIV-positive people, medical management of the associated complications is becoming more complex. This complexity is compounded by the growing number of drugs available to treat people with HIV/AIDS. Information regarding the adverse reactions and/or interactions of these drugs in combination is limited. The purpose of this paper is to review what is currently known about synergistic, antagonistic, and potentially toxic interactions. Included is a table containing side effects and interactions among drugs commonly used to treat clients with HIV/AIDS.

Effect of herpes simplex virus thymidine kinase expression levels on ganciclovir-mediated cytotoxicity and the "bystander effect"

Transfer of the herpes simplex virus type-1 thymidine kinase (HSV-tk) gene into tumor cells followed by ganciclovir (GCV) administration, will provide selective tumor cell killing. We studied the effect of herpes simplex virus thymidine kinase (HSV-tk) expression level on the HSV-tk/GCV-mediated "bystander effect." Clones of HSV-tk-transduced rat glioma cells (9L) were isolated that stably expressed with different levels of HSV-tk. All clones studied had similar sensitivity to ganciclovir with IC50 values ranging from 0.45 to 1.3 microM. Within certain enzyme level thresholds, in vitro evaluation of the bystander effect has shown that clones with higher level of HSV-tk expression exhibited a better bystander effect. Interestingly, the bystander effect was observed between different cell types. Both the transduction efficiency and bystander effect are essential factors for the success of the antitumor effect by the HSV-tk/prodrug GCV suicide gene system.

Treatment of experimental human mesothelioma using adenovirus transfer of the herpes simplex thymidine kinase gene

OBJECTIVE

The authors demonstrate the ability of an adenovirus vector expressing the herpes simplex thymidine kinase (HSVtk) gene to treat human malignant mesothelioma growing within the peritoneal cavity of severe combined immunodeficient (SCID) mice.

BACKGROUND DATA

Introduction of the HSVtk gene into tumor cells renders them sensitive to the antiviral drug ganciclovir (GCV). This approach has been used previously to treat experimental brain tumors. Although malignant mesothelioma is refractory to current therapies, its localized nature and the accessibility of the pleural space make it a potential target for a similar type of in vivo gene therapy using adenovirus.

METHODS

An adenovirus containing the HSVtk gene (Ad.RSVtk) was used to transduce mesothelioma cells in vitro. These cells were then injected into the flanks of SCID mice. Ad.RSVtk was also injected directly into the peritoneal cavity of SCID mice with established human mesothelioma tumors. Mice were subsequently treated for 7 days with GCV at a dose of 5 mg/kg.

RESULTS

Mesothelioma cells transduced in vitro with Ad.RSVtk formed nodules when injected in the subcutaneous tissue. These tumors could be eliminated by the administration of GCV, even when as few as 10% of cells were transduced to express HSVtk (bystander effect). Administration of Ad.RSVtk into the peritoneal space of animals with established multifocal human mesothelioma followed by GCV therapy resulted in the eradication of macroscopic tumor in 90% of animals and microscopic tumor in 80% of animals when evaluated after 30 days. The median survival of animals treated with Ad.RSVtk/GCV was significantly longer than that of control animals treated with similar protocols.

CONCLUSION

These results indicate that an adenoviral vector containing the HSVtk gene is effective in treating established malignant mesothelioma in an in vivo setting and raise the possibility of using adenovirus transfer of HSVtk for clinical trials in mesothelioma and other localized tumors.

Differential sensitivity of thoracic malignant tumors to adenovirus-mediated drug sensitization gene therapy

Malignant mesothelioma may prove to be an attractive candidate for somatic gene therapy with replication-deficient recombinant adenovirus transfer of a toxic, or drug sensitization gene. Transfer of the herpes simplex thymidine kinase type I gene (HSVtk), followed by exposure to the acyclic nucleoside drug ganciclovir, has been shown to be an effective tumor cell killing system. To study generalized applicability, we tested a number of thoracic malignant cell lines for their sensitivity to gancyclovir after infection with an adenoviral vector containing the HSVtk gene (Ad.RSVtk). Using the concentration of gancyclovir required to kill 50% of the cells (IC50) as a measure of sensitivity, we detected variable sensitivity among cell lines, with mesothelioma most sensitive (IC50 = 0.075 to 2.8 mumol/L gancyclovir), and non-small-cell carcinoma lines having an intermediate sensitivity (IC50 = 1.5 to 100 mumol/L). In contrast, an ovarian carcinoma line was extremely resistant (IC50 > 2000 mumol/L). To study the possible mechanisms for these differences, we studied cell lines with regard to their ability to be infected with an adenoviral vector containing a marker gene (Ad.CMVlacZ) and expression of the vitronectin receptor alpha v (an integrin cell adhesion molecule shown to be required for adenovirus internalization after initial binding). We found that the degree of lacZ transduction correlated with HSVtk sensitivity, whereas vitronectin receptor expression did not, suggesting that differences in initial viral binding ability, rather than internalization, may explain the sensitivity differences seen in vitro.

Ganciclovir transfer by human placenta and its effects on rat fetal cells

Cytomegalovirus is a common cause of intrauterine infection. Ganciclovir is an accepted therapeutic agent for this infection, but is proscribed in pregnancy, except when there is a life-threatening maternal infection, because of its known teratogenic and embryotoxic effects in experimental animals. There are no such data in humans and the human transplacental transfer of this drug has not been studied. This study defines the rate and mechanism of human-placental ganciclovir transport using maternal-facing syncytiotrophoblast vesicles and the perfused, isolated single-cotyledon system and determines further the effects of ganciclovir on fetal tissue, using cultured rat fetal hepatocytes. Ganciclovir was taken up by the maternal-facing placental membrane by a carrier-dependent, Na-independent system inhibited by adenine, guanine, and acyclovir, but not by cytosine and thymine or thymidine and uridine. By contrast, the overall transfer of the drug by the placenta was passive and without drug metabolism. Therefore, the drug is concentrated initially at the maternal placental surface and then crosses passively into the fetal compartment, with the latter process being rate-limiting. There was little or no toxic effect of high concentrations of ganciclovir on cultured fetal-rat hepatocytes.