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Keywords:

  • interferon alfa-2b;
  • ribavirin;
  • hepatitis C;
  • HCV-RNA;
  • neopterin;
  • 2′5′-oligoadenylate synthetase;
  • pharmacokinetics;
  • drug interaction;
  • pharmacodynamics

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Aims The primary objective of this study was to determine whether pharmacokinetic interactions occurred between interferon α-2b (IFN) and ribavirin in patients with chronic hepatitis C infections. Additionally this study assessed the single and multiple-dose pharmacokinetics of ribavirin and IFN, and compared the safety, tolerability and antiviral pharmacodynamics of IFN plus ribavirin compared with either drug alone.

Methods In this open label parallel group study, patients with chronic hepatitis C were randomized to receive IFN 3 million IU thrice weekly s.c. alone, ribavirin 600 mg twice daily p.o. alone or both drugs in combination over 6 weeks. Single and multiple dose pharmacokinetics and indices of antiviral pharmacodynamics were assessed during weeks 1 and 6, along with safety assessments during the study.

Results The range of mean ribavirin terminal phase half-lives after single doses was 44–49 h. Comparison of week 1 and week 6 AUC(0,12h) values showed accumulation in plasma of approximately 6-fold. The range of mean washout half-lives after week 6 was 274–298 h, reflecting release of ribavirin from deep compartment stores. The range of single and multiple dose IFN terminal phase half-lives was 5–7 h. IFN demonstrated an increase in bioavailability (∼2-fold) upon multiple dose administration. Ribavirin and IFN pharmacokinetic parameters for combined ribavirin and IFN were similar to those during monotherapy with either compound, although the power of this study to detect differences was low. Serum HCV-RNA titers and ALT concentrations were reduced by IFN alone, ribavirin alone reduced ALT concentrations only, and combined IFN plus ribavirin produced numerically greater falls in both measurements than either treatment alone. Serum concentrations of neopterin and activity of 2′,5′-oligoadenylate synthetase (2′5′-OAS) were increased by IFN alone and in combination with ribavirin, whereas serum 2′5′-OAS activity was decreased and neopterin concentrations unaltered by ribavirin monotherapy. IFN and ribavirin monotherapy produced characteristic changes in safety laboratory tests (IFN—reductions in white cells, neutrophils and platelets; ribavirin—reduced haemoglobin) and characteristic adverse event profiles (IFN—headache, flu-like symptoms, fatigue, anorexia, nausea, myalgia, and insomnia; ribavirin—headache, fatigue, myalgia, and pruritus). There was no additive effect of combination therapy on safety laboratory tests or reported adverse events. All changes were fully reversible upon treatment cessation.

Conclusions There was no evidence of pharmacokinetic interactions between IFN and ribavirin in this study. There were numerical trends indicating that the combination of IFN and ribavirin reduced titers of HCV-RNA to a greater extent than did either treatment alone, and the safety profile of combination therapy was similar to those of both monotherapy treatments.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The combination of the nucleoside analog ribavirin with interferon α-2b (Intron®A; IFN) has been shown in several open label studies [ 1[2]–3] and one double-blind study [ 4] to have improved antiviral efficacy compared with IFN alone in patients with chronic hepatitis C. In the latter study, 36% of patients treated with IFN 3 million international units thrice weekly (MIU TIW) plus ribavirin 1000–1200 mg day−1 for 6 months were negative for hepatitis C virus RNA (HCV-RNA) at 6 months post-treatment follow-up compared with 18% of those treated with IFN. The combination of IFN plus ribavirin has recently been approved in the USA for the treatment of chronic hepatitis C in patients who have relapsed following prior IFN therapy.

The pharmacological safety of this regimen requires evaluation, however. The present study was designed to address several important issues associated with this combination treatment regimen. First, IFN has been reported to inhibit activity of hepatic drug metabolizing enzymes, including cytochrome P450 [ 5, 6]. Although the exact nature of ribavirin elimination in man has not been determined, metabolism has been proposed as a principal route [ 7]. Therefore it is possible that IFN might affect the pharmacokinetics of ribavirin by inhibiting its metabolism. There is also no information on whether ribavirin might affect the pharmacokinetics of IFN. The primary objective of this study was thus to examine whether there were pharmacokinetic interactions between the two drugs. A multiple dose study design was used as IFN-induced inhibition of enzyme activity is not an immediate phenomenon, and ribavirin takes at least several weeks to achieve steady-state [ 8]. In addition, the present study described the single and multiple dose pharmacokinetics of ribavirin and IFN. The safety profiles and pharmacodynamic profiles [serum HCV-RNA titers, serum activity or concentrations of effector proteins, which may provide in vivo indices of antiviral activity (e.g. 2′5′ oligoadenylate synthetase (2, 5-OAS) and neopterin)] were also compared in patients on ribavirin and IFN alone or in combination.

Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

This was a single and multiple dose, open label, three arm parallel group, safety, pharmacokinetic and pharmacodynamic study in patients with chronic hepatitis C. The study protocol was approved by Ethics Committees at the Royal Free, Charing Cross and Middlesex Hospitals, and all patients provided written informed consent prior to participation. Thirty-eight male and female patients with compensated chronic hepatitis C infection, between the ages of 18 and 58 years participated in the study. Patients were either naive to prior IFN treatment (n=33) or could have relapsed following prior IFN treatment (n=5). They were selected from outpatient clinics at the Royal Free and Charing Cross Hospitals, London, UK. Patients were determined to be eligible to participate in the study on the basis of being positive for HCV-RNA by bDNA (Chiron) or PCR (National Genetics Institute) assays, having a recent liver biopsy consistent with chronic hepatitis C, abnormal ALT values (if available), and evidence of compensated liver disease, as manifest by normal prothrombin time, normal serum albumin and total bilirubin concentration. Additionally creatinine and fasting blood sugar had to be normal, with normal haematological and thyroid function parameters. Titers of anti-nuclear, anti-smooth muscle, anti-LKM, anti-microsomal and anti-thyroid antibodies were all less than 1:160. Patients were required to be negative for interferon neutralizing antibodies, anti-HIV (if available) and HBsAg, and to have normal alpha fetoprotein levels. Patients were excluded if they had significant other medical or psychiatric illness, substance abuse, other causes for liver disease than chronic hepatitis C, evidence of advanced liver disease, or hypersensitivity to IFN alfa or ribavirin.

Patients were randomly allocated to one of three treatment groups (A, B or C): Group A: IFN alone; Group B: ribavirin alone; and Group C: IFN plus ribavirin. Treatment was administered according to the following schedule: on day 1, week 1, patients were administered a single dose of treatment (i.e., IFN 3 MIU, ribavirin 600 mg or both) at approximately 08.00 h, followed by pharmacokinetic assessment. On days 2 through 7 of week 1 no other treatments were administered. From day 1 week 2 until day 1 week 6, patients received their allocated treatment (ribavirin 600 mg twice daily, IFN 3 MIU three times weekly or the combination). The last dose was administered at approximately 08.00 h on day 1 week 6, followed by pharmacokinetic assessment. Ribavirin dosing before the single and multiple dose pharmacokinetic assessment was performed after an overnight fast, as food has been shown to increase the bioavailability of ribavirin. Compliance with study treatments was assessed by questioning of patients and counting of capsules and/or unused vials during clinic visits. From weeks 7 through 10, there was a follow-up period of 4 weeks, during which no study medication was administered. Patients who completed the full 10 weeks of study and follow-up were eligible to enter a separate compassionate continuation study, and were treated with the above IFN and ribavirin dose regimen for a further 24 weeks.

Pharmacokinetic assessment of ribavirin and/or IFN was determined from fasted blood samples obtained during weeks 1 and 6. For patients receiving ribavirin (Groups B and C), 3 mL blood samples were obtained pre-dose (0 h), then at 1, 2, 3, 4, 6, 8, 10, 12, 14, 16, 24, 30, 36, 48, 72, 96, 120, and 168 h postdose. During weeks 2 through 5, additional blood samples were taken at 168 h, and during follow-up (weeks 7 through 10) samples were obtained once weekly for measurement of ribavirin concentrations. To reduce the possibility of haemolysis, ribavirin samples were collected into a syringe, the needle was removed and the contents of the syringe gently placed into a heparinized Vacutainer tube which had its top removed. The sample was gently mixed and stored on ice until centrifuged at 4° C and 1000 g for 15 min. The plasma was removed and frozen to at least −70° C until assayed.

For patients receiving IFN (Groups A and C) 5 ml blood samples were collected pre-dose, and at 2, 4, 6, 8, 10, 12, 14, 16 and 24 h post dose in weeks 1 and 6 for assessment of IFN pharmacokinetics. No additional samples were collected during weeks 2–5 or 7–10. Samples were collected into non-heparinized Vacutainer tubes, and the blood allowed to clot for 30 min at room temperature. The samples were then centrifuged for 15 min at 4° C and 1000 g, the serum was collected and frozen at −70° C until assayed.

Single blood samples were collected in all patients for measurement of serum neopterin concentrations, 2,5-OAS activity (predose on day 1, weeks 1 and 6) and for HCV-RNA titers (at screening and weekly during the study (weeks 1–10)). Samples were collected into non-heparin tubes, allowed to clot for 30 min at room temperature, centrifuged at 1000 g for 15 min at 4° C, and frozen at −70° C until assayed.

Serum concentrations of IFN were determined by a solid phase enzyme immunoassay (Anawa Laboratories, Zurich, Switzerland). The assay incubated samples with beads coated with two monoclonal antibodies which are sensitive to different epitopes of IFN, and one of these was also conjugated to peroxidase. After incubation and washing, the amount of peroxidase linked immunologically to the beads was determined by addition of a peroxidase substrate system tetramethylbenzindine/H2O2, and the coloured reaction product (which is proportional to IFN concentrations) was determined spectrophotometrically at 453nm. The assay limit of quantitation (LOQ) was 10 IU ml−1. Plasma concentrations of ribavirin were determined using a validated high performance liquid chromatography/mass spectrometric method using 13C-ribavirin as an internal standard (LOQ 50 ng ml−1 ). Separation was performed using a 3 μm Hypersil column (3 cm×4.6 mm) and a mobile phase of acetonitrile (82%) and ammonium acetate (18%). Intra- and inter-assay coefficients of variation were both <8%. Serum HCV-RNA titers were determined by a quantitative PCR method (assay sensitivity 100 copies ml−1; National Genetics Institute, Culver City, California). Serum neopterin concentrations and 2′5′-OAS activity were measured by commercially available competitive EIA and radioimmunoassay methods respectively (neopterin: ImmuchemTM Microwell Neopterin Enzyme Immunoassay Kit, ICN, CA, USA; 2′5′-OAS: Eiken Chemical Co, Tokyo, Japan).

Individual plasma ribavirin and serum IFN concentration-time data above the LOQ were used for pharmacokinetic analysis using model-independent methods [ 9]. The maximum concentration (Cmax ), time of maximum concentration (tmax ) and the final quantifiable sampling time (0-t ) during weeks 1 and 6 were the observed values. The area under the plasma concentration-time curve from time zero to the final timepoint [AUC(0,t )], and over a dosing interval, τ, (AUCτ ) were calculated using the linear trapezoidal method.

Safety assessments included recording of vital signs (systolic and diastolic blood pressure, heart rate, oral body temperature and respiratory rate) and ECGs at screening, and throughout the study and followup periods. Complete blood count, clinical chemistry and urinalysis tests were obtained at screening and weekly during study and followup periods. Patients were observed and questioned throughout the study for the occurrence of adverse events.

Pharmacokinetic parameters were analyzed using analysis of variance (ANOVA) extracting the treatment effect (IFN: Group A vs C; ribavirin: Group B vs C). The analyses were done for the AUCs and Cmax in both original and log scales and for other pharmacokinetic parameters in the original scale. The power to detect a 20% differences in means were also calculated for AUCs and Cmax. For a study with 12 patients per group, and assuming a coefficient of variation of approximately 15% for IFN AUC (Schering-Plough Study I91–223, data on file), there was 81% power to detect a 20% difference between the combined treatment group (C) and the reference monotherapy group (A), α=0.05, two-tailed. Similarly, assuming a coefficient of variation of approximately 15% for ribavirin AUC [ 7] and 12 patients per group, there was 81% power to detect a 20% difference between the combined treatment group (C) and the reference monotherapy group (B), α=0.05, two-tailed.

To assess whether steady-state conditions were reached for ribavirin, trough concentrations during weeks 3, 4 and 5 for Groups B and C were analyzed using a repeated measures ANOVA extracting effects associated with patient and week. If a significant difference among the weeks was indicated, regression analyses were used to determine if there was a significant linear trend without significant deviation from linearity. When these conditions were met, an estimate of slope (average change/week) was provided. Serum 2′5′ OAS activity, neopterin concentrations and log-transformed HCV-RNA titers were analyzed using ANOVA extracting the treatment effect. The changes from predose week 1 to Day 1 week 6 were analyzed using the same model. Each pair of means was compared using the residual error from the analysis of variance model. These data were also analyzed using the non-parametric Kruskal-Wallis/Wilcoxon tests.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Thirty-eight healthy male and female patients with chronic hepatitis C (24M, 14F, mean age 38.7 years, range 18–58 years; mean weight 73.2 kg, range 48–94 kg) participated in this study and 36 completed the study as planned. Patients 38 (combined IFN plus ribavirin) and 33 (IFN alone) withdrew from the study on day 1 week 1 and week 4, respectively, because of treatment-related adverse events.

Single and multiple dose pharmacokinetics of IFN and ribavirin

Following single dose administration of IFN 3MIU s.c., mean serum concentrations peaked at 7 h, and rapidly decreased to undetectable concentrations by 17 h in both groups A and C ( Figure 1). Serum concentrations were undetectable predose, week 6. After dosing in week 6, mean serum IFN concentrations peaked at 5–7 h, and were undetectable by 20–21 h ( Figure 1). An increase in apparent bioavailability was noted, with the ratio of Week 6 to Week 1 AUC(0,t ) and Cmax of 2.0 and 1.7, respectively ( Table 1).

image

Figure 1. Mean single and multiple dose serum IFN concentrations after 3MIU doses s.c.: IFN monotherapy single dose (▵) and multiple dose (▿); IFN plus ribavirin single dose (▴) and multiple dose (▾).

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Table 1.  Mean (%CV) IFN and ribavirin single and multiple dose pharmacokinetic parameters, and statistical comparison of log-transformed pharmacokinetic parameters. Thumbnail image of

After administration of a single oral 600 mg dose of ribavirin there was rapid absorption with a tmax of approximately 2 h, followed by rapid distribution and long terminal phases ( Figure 2), with a mean final concentration timepoint at approximately 100 h in both groups B and C. Following multiple dosing in weeks 2–5, trough concentrations began to asymptote by 3–4 weeks of ribavirin administration. Steady state assessment from the trough concentrations in weeks 3, 4 and 5 (i.e., after 2, 3 and 4 weeks of multiple dose administration; see Table 2, Figure 3) indicated significance (i.e., lack of steady state) for Group C (IFN plus ribavirin) but not Group B (ribavirin monotherapy). However the general trend for trough concentrations in both groups was asymptotic and was also characterized by high variability for individual values. The multiple dose pharmacokinetics of ribavirin (day 1 week 6) again showed rapid absorption and distribution phases over the 12 h dosing interval for both groups B and C ( Figure 2), with mean tmax range of 2–3 hrs. Mean peak to trough fluctuation over the dosing interval was approximately 50%. Accumulation could not be formally assessed in that it was not possible to determine AUC(I) following single dosing. However, comparison of week 1 and week 6 AUC(0, 12 h) indicated an accumulation factor of approximately 6. The washout elimination of ribavirin was determined in detail during week 6 and from the single weekly concentration samples during weeks 7–10 ( Figure 3). The range of mean half-lives following multiple dosing was 274–298 h, and there were measurable concentrations in all patients at the end of week 10.

image

Figure 2. Mean single and multiple dose plasma ribavirin concentrations after 600 mg doses p.o.: monotherapy single dose (▵) and multiple dose (▿); ribavirin plus IFN single dose (▴) and multiple dose (▾).

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Table 2.  Mean (%CV) trough plasma ribavirin concentrations, weeks 3–5. Thumbnail image of
image

Figure 3. Mean ribavirin trough concentrations (Ctrough) during treatment with ribavirin alone (▴) or in combination with IFN (▾).

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Assessment of pharmacokinetic interaction

To assess possible pharmacokinetic interactions between ribavirin and IFN, log-transformed AUC and Cmax values during weeks 1 and 6 were compared (IFN: Group A vs C; ribavirin: Group B vs C; Table 1). The estimate of relative bioavailability of ribavirin when administered in combination with IFN compared with ribavirin when administered alone ranged from 114–123 (week 1) to 104–113 (week 6); for IFN these estimates were 118–125 for week 1 and 80–92 for week 6. Ninety percent confidence intervals for all of these comparisons included 100, although none of the confidence intervals met the criteria for bioequivalence (80–125%). The power to detect a 20% difference between groups ranged between 10–26% for week 1 data, and 15–29% for week 6 data.

Pharmacodynamic assessment

Mean serum concentrations of neopterin were increased at the beginning of week 6 relative to week 1 in Group A (IFN alone: 210%) and Group C (IFN plus ribavirin: 270%). Patients in Group B, who received ribavirin alone, had mean week 6 neopterin values approximately 87% of week 1 ( Table 3). Mean serum 2′5′ OAS activity was also increased at the beginning of week 6 relative to week 1 in Group A (IFN alone: 463% of week 1) and in Group C (IFN plus ribavirin: 550% of week 1), but fell in patients in Group B (ribavirin alone: 10% of week 1) (Table 4).

Table 3.  Mean (s.d.) concentrations of effector proteins. Thumbnail image of

Mean serum HCVRNA concentrations during the study are shown in Figure 4. Treatment with ribavirin produced a 0.39 log reduction in mean HCVRNA concentrations by the beginning of week 6; IFN produced a mean 1.41 log reduction, and combination treatment produced a mean 2.97 log reduction. Comparison of change scores (the difference between week 6 and week 1 titers) showed statistically significantly differences between ribavirin monotherapy and IFN monotherapy group (P=0.009) and ribavirin monotherapy and combination treatment groups (P<0.001). Although a numerically greater fall occurred in the combination treatment group compared with IFN monotherapy, this was not statistically significantly different (P=0.11). Within 1–3 weeks of stopping treatment, HCV-RNA titers returned to baseline values in all patients. Mean serum ALT concentrations were reduced in all three treatment groups by the beginning of week 6 (ribavirin monotherapy: 37% reduction; IFN monotherapy: 48% reduction; IFN plus ribavirin: 71% reduction), and returned to baseline within 2–4 weeks of stopping study treatments ( Figure 4).

image

Figure 4. Mean serum HCV-RNA titres (a) and ALT concentrations (b) during and following treatment with IFN alone (○), ribavirin alone (□), or combined IFN plus ribavirin (▵).

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Safety assessment

Treatment with IFN alone produced falls in white cells, neutrophils and platelets, but no change in haemoglobin concentrations, while treatment with ribavirin alone reduced haemoglobin concentrations, and either did not change or increased neutrophil and platelet counts ( Figure 5). Combined treatment with IFN and ribavirin produced falls in haemoglobin, white cells and neutrophils of similar magnitude to those of the monotherapy groups, however the reduction in platelets during combined treatment was less than that in the IFN monotherapy group ( Figure 5). There were no changes in other haematological, biochemical or ECG parameters of clinical relevance during the study. Changes in white cell counts were almost entirely accounted for by the decrease in neutrophils; lymphocyte counts were essentially unchanged in any treatment group. No patient required dose adjustment in response to haematological changes during the study. All patients reported adverse events during the study. Those treated with IFN (Groups A and C) had generally similar profiles of reported symptoms, most commonly headache, flu-like symptoms, fatigue, anorexia, nausea, myalgia, and insomnia. Those treated with ribavirin (Groups B and C) also had similar symptom profiles, including headache, fatigue, myalgia, and pruritus. The majority of adverse events reported were of mild-moderate severity. Severe adverse events were reported in the three treatment groups as follows: IFN monotherapy: four reports of headaches, a single report of musculoskeletal pain, myalgia and insomnia; ribavirin monotherapy: two reports each of headache and fatigue, and single reports of paresthesia, pharyngitis and viral infection; IFN plus ribavirin: four reports of headache, two reports each of flu-like symptoms and vomiting and single reports of somnolence and tooth disorder.

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Figure 5. Mean neutrophil, (a) haemoglobin (b) and platelet (c) values during and following treatment with IFN alone (○), ribavirin alone (□), or combined IFN plus ribavirin (▵).

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

This study provided the first detailed description of multiple dose pharmacokinetics of IFN and ribavirin in man. Treatment-related differences in early pharmacodynamic responses were also identified in this study. There was no evidence of pharmacokinetic interactions between IFN and ribavirin under single or multiple dose conditions, although the power to detect an interaction was low.

The design of the present study was influenced by the pharmacokinetic and clinical profiles of IFN and ribavirin. A multiple dose design was required as the inhibitory effects of IFN might not have been evident in a single dose study, and ribavirin takes at least several weeks to reach steady state. A treatment duration of 4 weeks was chosen in order for plasma ribavirin concentrations to be at steady state, based on previously published data [ 8], and a longer duration of ribavirin monotherapy treatment was not considered because it was previously shown that ribavirin monotherapy does not affect serum HCV-RNA levels while producing anaemia [ 10], with ensuing ethical issues about administering ribavirin for longer periods without any potential clinical benefit. Because ribavirin accumulates extensively in red cells and other tissue compartments and is cleared slowly [ 11, 12] a crossover study design was not feasible and thus a parallel group design was chosen. Ribavirin has teratogenic effects and both compounds have hematological side effects, so it was decided that this study could only be carried out in patients with a clinically relevant condition (i.e. chronic hepatitis C). Estimates of pharmacokinetic variability which were used to size the study were lower than the variability observed in the present study, so that the power of this study to show a pharmacokinetic interaction was low.

Comparison of the mean single and multiple-dose pharmacokinetic parameters and concentration-time profiles for both compounds showed no statistically significant differences for the combination treatment group (C) compared with either monotherapy group (A or B). It should be noted that the statistical power of this study was low (IFN: 15–26%, ribavirin: 10–29%, to detect a 20% difference in means). Both the parallel group study design and the high pharmacokinetic variability of IFN and ribavirin contributed to the low statistical power. This also means that one method of assessing interactions (by using bioequivalence criteria (e.g., [ 13]) was inappropriate for this study due to the wide confidence intervals for all parameters. Despite the low power from the present multiple dose study, it does not appear that there is a significant pharmacokinetic interaction between IFN and ribavirin. However because of the wide confidence intervals, more subtle pharmacokinetic interactions might not be detected in this study. These findings are consistent with a recent study which showed that IFN did not alter the AUC or Cmax of the nucleoside analog reverse transcriptase inhibitor didanosine [ 14]. These findings also suggest that the mechanism for the enhanced antiviral activity of combined IFN and ribavirin compared with either drug alone is not due to an alteration of the pharmacokinetics of one or both compounds as a result of a drug interaction.

This study provides the first detailed description of single and multiple dose pharmacokinetics of ribavirin and IFN in man. The single dose concentration- time profile of ribavirin is identical to that reported previously [ 7, 15] with rapid absorption and distribution phases, and a long terminal elimination phase. Ribavirin accumulation after 4 weeks of dosing, assessed by comparing AUC(0, 12 h) values, was approximately 6 fold. This study indicates that ribavirin plasma concentrations are close to, but not at steady state after four weeks of therapy. However, mean trough concentration profiles of both Groups B and C were similar at the end of weeks 3, 4 and 5, and the general trend for both groups was asymptotic. The mechanism for ribavirin accumulation in red blood cells has recently been determined [ 16], and is via an es nucleoside transporter [ 17, 18]. As es transporters are distributed on many cell types in man, this would account for the extensive accumulation in all tissue compartments, and the high Vdss estimates (>1000 l; [ 7]). This would also explain the observed long washout half-life after multiple dosing because of slow elimination from a ‘deep compartment’.

Multiple dose IFN pharmacokinetic parameters and concentration-time profiles were increased compared with single dose results (AUC and Cmax were increased approximately 2 fold). However this was probably not due to accumulation (week 6 pre-dose IFN concentrations were unmeasurable), different absorption characteristics (tmax values were similar for single and multiple doses), or rate of elimination (the terminal phase slopes for single and multiple doses were similar). It was not due to an interaction with ribavirin, as changes of a similar magnitude were observed in patients treated with IFN alone and those who received combination therapy. One possible mechanism to account for the increased bioavailability of IFN after multiple dosing could involve changes in IFN receptor density. Following multiple dose treatment, IFN receptors are reduced by approximately 50% [ 19]. Subcutaneously administered IFN reaches the systemic circulation via the lymphatic system, with a proportion binding to IFN receptors on lymphatic immunocytes. With reduced IFN receptor numbers after multiple dose therapy, a larger proportion of an administered dose of IFN would reach the systemic circulation, leading to increased exposure.

Although not a primary objective, this study provided preliminary data on the pharmacodynamic profiles of IFN and ribavirin alone and in combination, including changes in serum neopterin concentrations, activity of the enzyme 2′5′-OAS, HCV-RNA concentrations and ALT concentrations. Interferons cause induction of 2′5′-OAS, an enzyme which is central to their antiviral activity [ 20], and monocyte activation by interferons increases neopterin synthesis [ 21], and these changes can be used as evidence of in vivo biological activity of interferons [ 22]. IFN alone increased serum neopterin concentrations and 2′5′-OAS activity as has been previously demonstrated [ 22[23]–24], whereas ribavirin did not change neopterin concentrations and decreased 2′5′-OAS activity. The combination of IFN and ribavirin produced changes in effector protein concentrations of similar magnitude to IFN alone. The observation that these changes after combination treatment were similar to those after IFN treatment alone suggests that the enhanced antiviral activity of the combination treatment was not due to increased synthesis or activity of effector proteins. There was a substantial fall in HCV-RNA concentrations during 4 weeks of treatment with IFN, and a much smaller change in patients treated with ribavirin monotherapy. Combination IFN plus ribavirin produced greater mean reductions in HCV-RNA titers than both monotherapy groups. This was statistically significant compared with ribavirin monotherapy treatment, but not statistically significantly different compared with IFN monotherapy. It is possible that the high variability of these data, the heterogeneity of the patient population (not controlled for pretreatment genotype, baseline viral load, or prior IFN treatment history) and the small number of patients in each treatment group may have made it difficult to detect differences between IFN monotherapy and combination groups. As the addition of ribavirin to IFN is primarily thought to increase treatment response rates via reduction in relapse, the suggestion of enhanced rates of early viral eradication in the present study are of interest and warrant further study.

The safety and tolerability profiles of IFN and ribavirin were consistent with those previously reported. IFN produced characteristic flu-like and other somatic symptoms, and reductions in white cell, neutrophil and platelet counts, while ribavirin produced mainly headache, fatigue and myalgia, and a drop in haemoglobin. The combination of IFN and ribavirin produced changes in safety laboratory tests which were similar to those of both drugs alone and there were no additive toxicities during this short period of dosing. No dose reductions were required by any patient for these hematological changes. Similarly, adverse event reports and severities were consistent with those of either drug alone, but with no evidence of additive effects. All changes were completely reversible upon cessation of study treatments.

In conclusion, no evidence of a pharmacokinetic interaction was observed between IFN and ribavirin in this study. There were numerical trends indicating that the combination of IFN and ribavirin reduced concentrations of HCV-RNA to a greater extent than did either treatment alone, with a safety profile that was similar to those of both monotherapy treatments. The greater decline in HCV-RNA titers by combined IFN and ribavirin treatment could not be explained by differences in pharmacodynamic or pharmacokinetic measures assessed in this study.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The assistance of Drs Shiela Jacobs and Robert Clement, Mss Josephine Lim, Anita Durso, Maggi Salfi and Mr Murali Tatanaya (SPRI), Mrs Ruth Jacobs (Royal Free Hospital), and John Turner, Javid Sabir, Gerry Lenihan and Hassi Devalia (LAB Clinical Trials Unit, Churchill Clementine Hospital) is gratefully acknowledged.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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