The effect of sorivudine on dihydropyrimidine dehydrogenase activity in patients with acute herpes zoster*

The effect of sorivudine on dihydropyrimidine dehydrogenase activity in patients with acute herpes zoster

Objective: Bromovinyl-uracil (BW) is the principal metabolite of sorivudine, a potent anti-zoster nucleoside. BW binds to, and irreversibly inhibits, the enzyme dihydropyrimidine dehydrogenase (DPD). The objective of this study was to assess the time course of recovery of DPD activity after oral administration of sorivudine in patients with herpes zoster and to correlate restoration of DPD activity and levels of uracil with the elimination of sorivudine and its metabolite BW from the circulation. Methods: Sorivudine was given orally as 40 mg once-daily doses for 10 consecutive days to a total of 19 patients with herpes zoster. Serum sorivudine, BW, and circulating uracil and DPD activity in peripheral blood mononuclear cells (PBMCs) were determined before, during, and after administration of sorivudine. Results: BW was eliminated from the circulation within 7 days after the last sorivudine dose. DPD activity in PBMCs, which was completely suppressed in 18 of the 19 subjects and markedly suppressed in the remaining subject during administration of sorivudine, recovered to baseline levels within 19 days after the last dose of sorivudine in all subjects and within 14 days in all but one of the subjects. The restoration of DPD activity was temporally associated with elimination of BW from the circulation. The elevated uracil concentrations produced by inhibition of DPD activity fell rapidly after cessation of sorivudine administration and also were temporally associated with elimination of BW from the circulation. The time course of recovery of DPD activity in three patients with renal impairment was similar to that of the other subjects. Conclussions: This study indicates that sorivudine therapy is associated with a profound depression of DPD activity. Recovery of DPD activity occurred within 4 weeks of the completion of sorivudine therapy, which indicates that fluorinated pyrimidines may be safely administered 4 weeks after completion of sorivudine therapy. (Clin Pharmacol Ther 1997;61:563-73.)

Jieming Yan, MD, Stephen K. Tyring, MD, PhD, Monica M. McCrary, MD, Patricia C. Lee, MD, Stephen Haworth, MD, Ralph Raymond, MS, Steven J. Olsen, MS, and Robert B. Diasio, MD Bivvnin~bam, Ala., Galveston, Texas, and l+inceton, N.J.

From the Division of Clinical Pharmacology, Department of Pharmacology and Toxicology, University of Alabama at Bir- mingham, Birmingham; the Department of Dermatology and Microbiology/Immunology, University of Texas Medical Branch, Galveston; and Bristol-Myers Squibb Pharmaceutical Research Institute, Princeton.

Supported by Bristol-Myers Squibb Pharmaceutical Research Institute.

Received for publication Sept. 10, 1996; accepted Dec. 3, 1996. Reprint requests: Robert B. Diasio, MD, Department of Phar-

macology and Toxicology, Box 600, Volker Hall 101, University of Alabama at Birmingham, Birmingham, AL 35294.

Copyright 0 1997 by Mosby-Year Book, Inc. 0009-9236/97/$5.00 + 0 13/l/79691

Sorivudine (BV-araU, SQ32,756, l-B-o-arabino- furanosyl-E-5-[2-bromovinyl]uracil) is a synthetic analog of deoxythymidine nucleoside with antiviral activity against varicella-zoster virus,r herpes sim- plex virus type J2 and Epstein-Barr virus.3 It is a potent inhibitor of varicella-zoster virus, being 3000- fold more potent than acyclovir (INN, aciclovir) in vitro assay$ and can be administered as a single daily oral dose. Sorivudine has been underinvesti- gated for the treatment of localized herpes zoster infections in both immunocompromised and immunocompetent patients.

In general, sorivudine has been well tolerated in

563

564 Tan et&. CLINICAL PHARMACOLOGY &THERAPEUTICS

MAY 1997

patients who have received it worldwide in clinical trials outside Japan.4 In clinical trials in Japan, however, three deaths occurred in association with in- advertent coadministration of the chemotherapeutic drug tegafur (a 5-fluorouracil-containing com- pound) with sorivudine. After marketing of sorivudine in Japan in 1993, another 23 cases of severe bone marrow depression were reported following initiation of sorivudine therapy in patients receiving continuous oral 5-fluorouracil drug therapy; 16 of these patients died of complications of 5-fluorouracil toxicity.4,5 It is thought that the administration of sorivudine suppressed the catabolism of 5fluorouracil in these patients, resulting in higher levels of 5-fluorouracil than anticipated.

The principal metabolite of sorivudine in humans, bromovinyl-uracil (BVU), is a potent inhibitor of dihydropyrimidine dehydrogenase (DPD; EC 1.3.1.2).6 DPD, the initial and rate-limiting enzyme in pyrimidine catabolism, is the principal enzyme involved in degradation of 5-fluorouracil. In humans, more than 85% of administered 5-fluorouracil is catabolized by DPD.7,8 It has been shown that patients with inherited deficiencies in DPD activity are at increased risk of toxicity during 5-fluorouracil chemotherapy.‘-r4 Paradoxically, reduced pyrimidine metabolism may increase the availability of 5-fluorouracil and thus permit the use of lower doses.6,15,16

Patients with malignancies and lymphoprolif- erative disorders, who are candidates for fluoropyrimidine-based chemotherapeutic regi- mens, have a high risk of development of herpes zoster. Such patients would benefit from the addition of sorivudine to the available drug armamen- tarium if 5-fluorouracil toxicity caused by suppression of DPD activity by BVU could be avoided. The purpose of this study was to determine the extent and duration of suppression of DPD activity in adults with herpes zoster treated with oral sorivudine and thereby to determine when subsequent treatment with fluorinated pyrimidines could be safely administered. We determined blood levels of uracil and the time course of recovery of DPD en- zymatic activity after oral administration of sorivudine and correlated the levels of uracil and restoration of DPD activity with the elimination of sorivudine and BVU from the circulation. Because there is a significant linear correlation between DPD activity in the liver and in peripheral blood mononuclear cells (PBMCs),17 the DPD activity was measured in PBMCs obtained from 19 volunteers

with herpes zoster who received oral administration of sorivudine.

METHODS Study design and subject selection. This was an

open, nonrandomized, noncomparative study con- ducted in compliance with the Institutional Review Board of the University of Texas Medical Branch at Galveston. Written informed consent was obtained from each patient before the performance of any study-related procedures. Eligible subjects were either immunocompetent individuals 950 years old or immunocompromised individuals ~-18 years old with clinically apparent, active localized herpes zoster. Evidence of normal hepatic and renal function was required. Women were permitted to enroll in the study, provided they had a documented negative pregnancy test and were using a recognized reliable method of contraception during administration of study medication. The patient’s history was obtained before administration of the drug. In addition, a physical examination, pregnancy test (for women), and clinical laboratory tests (serum chemistry and hematology) were performed, and vital signs and symptoms were evaluated. The physical examination was repeated on days 5, 10, and 38 and the clinical laboratory tests were repeated on day 10.

Drug administration and blood sample collection. The subjects took an oral, 40 mg dose of sorivudine once daily for 10 consecutive days for treatment of acute local zoster. Subjects were instructed to take a single tablet (40 mg tablet) with 50 to 100 ml water at approximately 9 AM each day.

Blood samples for measurement of sorivudine and BW were obtained immediately before dosing on day 1 and on days 2, 5, 7, and 10 during sorivudine treatment. Additional samples were collected after the last dose of sorivudine on days 14, 17, and 21. Samples were collected with collection tubes, without anticoagulant. Clotting was allowed to occur for approximately 30 minutes, and then serum was obtained by centrifugation for 15 minutes and stored at -20” C until analysis.

Blood samples for DPD analysis were collected on day 1 (before the first dose of sorivudine) and on days 2, 5, 7, and 10 during sorivudine treatment. Additional samples were obtained on days 14, 17, 21, 24, 28, 31, 35, and 38 after the last dose of sorivudine. Because of the previously reported circadian variability of DPD activity in human PBMCs, all samples were taken at approximately the same time of day to minimize the effects of diurnal vari-

CLINICAL PHARMA COLOGY &THERAPEUTICS VOLUME 61. NUMBER 5 Tan et al. 565

ation on DPD activity.” PBMCs were obtained from blood samples (25 ml) drawn from a peripheral vein into evacuated collection tubes that contained heparin. After centrifugation at 5OOg for 30 minutes at 25” C over 15 ml of Histopaque, the PBMC frac- tion was removed carefully, washed three times with phosphate-buffered saline solution, and frozen at -70” C until assay. At the time of assay, the PBMCs were slowly thawed and then added to 300 l~,l buffer A, which contained 35 mmol/L potassium phosphate (pH 7.4), 2.5 mmol/L magnesium chloride, and 10 mmol/L 2-mercaptoethanol. The sample was placed on ice and sonicated three times for 10 seconds, with a 30-second interval between sonications. After centrifugation at 16,000g for 30 minutes at 4” C, the supernatant was removed and used in the subsequent enzyme assay. Protein was measured by use of the method of Bradford.”

Plasma and serum remaining after DPD and sorivudine and BVU assays were used to determine circulating uracil concentrations in 14 of the subjects. In the other five subjects, there was not enough plasma or serum collected to assay the uracil concentration.

Measurement of sorivudine and BVU. A validated assay based on HPLC analysis with ultraviolet de- tection was used for the determination of sorivudine and BW in human serum. A 50 l~,l of internal standard, carboxyvinylarabinofuranosyluracil (CV- araU) was added to 500 ~1 of each serum specimen quality control sample or assay standard, followed by 10 ml of chloroform-isopropanol (4:l). After shaking, the tubes were centrifuged for 10 minutes at 2700 rpm and the upper layer aspirated to waste. The organic phase was evaporated at 45” C under vacuum, the residue was reconstituted in 200 ~1 of the mobile phase, and a 25 ~1 aliquot was injected onto an alkylphenyl HPLC column (25 cm X 4.6 mm internal diameter; 5 km), preceded by a guard column (Brownlee RP-18, 1.5 cm X 4.6 mm internal diameter; 7 pm). The mobile phase was a mixture of 50 mmol/L ammonium acetate, acetonitrile, and methanol (78.5:12.5:10, vol/vol at pH 5.0) at a flow rate of 1.0 ml/min. The et&rent was monitored with an ultraviolet light detector at a wavelength of 295 nm. The nominal retention times were 8.0, 9.5, and 11.7 minutes for CV-araU, sorivudine, and BW, respectively. No endogenous interference peaks coeluted at the retention times of CVaraU, sorivudine, or BW. With use of 0.5 ml human serum, the limit of quantitation was 20 rig/ml for both sorivudine and BVU. Linearity (R 2 0.999) was shown for

sorivudine and BVU in the concentration range from 20 to 2000 r-&ml. The analysis of quality control samples provided precision estimates of ~7.6% relative standard deviation (RSD) for interassay and 52.7% RSD for intraassay variability. The pre- dicted concentration of quality control samples de- viated less than 5.5% from the respective nominal concentrations for both sorivudine and BW. Sorivudine and BW were stable in serum for at least two freeze-thaw cycles and were stable in the autosampler at room temperature for at least 24 hours. Each analytical run included a standard curve of sorivudine or BVU and three quality control specimens of the respective analyte, all of which were prepared daily. The criteria for acceptance of each analytical run included ? values of at least 0.990 for standard curves and an RSD for quality control specimens of no greater than 10%.

Measurement of uracil. Plasma and serum samples were deproteinated with perchloric acid at 0” C and adjusted to pH 6.0 to 6.5 with a mixture of 4 mol/L potassium hydroxide and 1 mol/L dipotassium hy- drogen phosphate. After filtration through a 0.2 Frn Supor- membrane filter, 100 ~1 of the supernatant was injected onto a Hewlett-Packard 1050 HPLC system equipped with a reversed-phase Hypersil ODS column (25 cm X 4.6 mm internal diameter; 5 m), preceded by a guard column (LiChroCART 100, RP-18, 5 pm). The mobile phase consisted of 10.0 mmol/L potassium phosphate and 5.0 mmol/L tetrabutylammonium hydro- gen sulfate (pH 6.4) at a flow rate of 1.0 ml/min. The eluant was monitored at 261 nm with a Hewlett- Packard 1050 variable wavelength detector. The retention time of uracil was 9.1 minutes with the lower limit of quantitation of 10 rig/ml. The extraction efficiency for uracil in both plasma and serum was 84.6%. The standard curve was linear over the concentration range of 0.1 to 10 pmol/L (Y” 2 0.999). The analysis of quality control samples provided precision intraassay estimates of ~4.1% RSD and interassay estimates of ~8.0% RSD. Uracil in the extracted supernatant was stable in the autosampler at room temperature for at least 12 hours.

Measurement of DPD activity in PBMCs. DPD enzyme activity was determined as previously reported.” The reaction mixture consisted of 200 pm NADPH, 20 pm [3H]5-fluorouracil, buffer A, and 250 pg of cytosolic protein obtained from the peripheral mononuclear cells in a final volume of 1 ml. The mixture was incubated at 37” C, and 175 ul aliquots of the reaction solution were removed at 5,

566 Yan et al.

Table I. Subject demographics Subject Age Height Weight

No. (yr) (cm) (kg) Sex Race

1 2 3 4 5t 6 7 8 9

10 11 12 13 14-t 15 16t 17t 18 19

Mean SD Range

65 164 27* 168 27* 168 43* 183 64 170 34* 173 73 160 59 164 45* 163 51 185 64 180 61 165 29* 170 77 165 65 178 45* 152 38* 158 62 152 57 165

52 167 15.7 9.3

22-77 152-185

54 Female White 53 Female White 92 Male White 83 Male White 83 Female White 81 Male White 64 Femeale Hispanic 66 Male Black 77 Female White 79 Male White

128 Male White 64 Female White 70 Male Hispanic 55 Female White 85 Male White 54 Female Hispanic 59 Female White 84 Female White 72 Female White

74 18.0

53-128

*Subjects <50 years old enrolled with trigeminal zoster. tImmunocompromised subjects: long-term steroid therapy (subjects 14,

16, and 17) or cyclosporine (INN, ciclosporin) therapy (subject 5).

10, 20,30, and 60 minutes and mixed with an equiv- alent volume of ice-cold ethanol to terminate the enzyme reaction. The sample was placed in a -70” C freezer for at least 10 minutes and then centrifuged at 16,000g for 10 minutes to remove precipitated protein. The supernatant was filtered through a 0.2 p,rn Acre filter (Gelman Sciences, Ann Arbor, Mich.) and the separation of 5fluorouracil and its catabolites was performed by an HPLC method we have described in detail previously.20 After HPLC analysis, the 5fluorouracil catabolites (products) at each time point were quantitated. The data were plotted with the products formed (y) versus time (x) to determine the slope of the curve by linear regression analysis. The slope was divided by the amount of protein added to obtain the final DPD activity expressed as nanomoles per minute per milligram of protein.

Statistical analysis. All statistical analyses were carried out with SAS/STAT version 6.09. Suppres- sion of DPD activity, duration of suppression after the cessation of sorivudine treatment, and time for DPD activity to return either to baseline levels or to levels ~0.064 nmol/min/mg were assessed. The lat-

CLINICAL P HARMACOLOGY &THERAPEUTICS MAY 1997

ter value is the lower limit of the 95th percentile for frozen PBMCs.13 The lower 95th percentile of DPD activity was chosen as patients with DPD activity in PBMCs below this range may be at risk of adverse reactions to 5-fluorouracil.13 The association of BVU serum concentration with suppression of DPD activity and the association of BVU serum concentration with circulating uracil concentrations were determined. Statistical tests were carried out at the one-tailed 5% significance level for DPD and at the two-tailed 5% significance level for uracil. Differ- ences from baseline (day 1) DPD activities were calculated for each subject, and a one-sided Student t test was performed on the mean differences, testing whether mean differences were significantly less than baseline. A statistically significant mean value would indicate a suppression of activity, whereas a nonsignificant value might indicate the return of DPD activity to baseline values. The relationship of circulating uracil to BW serum concentrations was assessed by means of correlation and regression analysis. In addition, a linear model that considered uracil concentration to be dependent on BW concentration and BVU-by-subject interaction was used to assess the relationship. A significant BVU-by- subject interaction would imply differing relationships across subjects. A partial correlation coefficient (partial r) adjusting for BVU, subject, and BVU-by-subject relationships was calculated by the sum of type I sum of squares for BVU and BVU- by-subject divided by this value plus the sum of squares error (partial ?). Individual subject corre- lations were calculated.

RESULTS A total of 19 subjects were enrolled in the study

(Table I), all of whom completed the study. A total of four subjects had mild clinical adverse events that were unlikely to have been related to administration of the study drug. There were no deaths or serious adverse events and no discontinuation because of an adverse event.

Serum sorivudine and BVV concentrations. The mean serum concentration-time profile of sorivudine after oral administration of 40 mg sorivudine once daily for 10 days is shown in Fig. 1. Sorivudine was last detected in the serum on day 14 (4 days after the end of therapy) in 18 of 19 subjects and on day 17 in the remaining subject (subject 14; Table II). Four of the subjects (subjects 7, 12, 14, and 15) had consistently elevated on-treatment serum concentrations of sorivudine that ranged from 562 to

CLINICAL I’ HARMACOLOGY & THERAPEUTICS VOLUME 61, NUMBER 5 Tan et al. 567

1400 -

1200 -

1000 -

800 -

600 -

400 -

200 -

0 a ’ I I I I 0 5 10 15 20

Day

Fig. 1. Mean serum sorivudine concentration after oral administration of 40 mg sorivudine once daily for 10 days.

Table II. Last day on which sorivudine and BW were detectable in serum Sorivudine BW

Study Day No. of Cumulative No. of Cumulative day after dosing subjects % of subjects subjects % of subjects

10 0 11 58 1 5 11 1 3 74 0 5 14 4 4 95 12 68 17 7 1 100 6 100

BW, Bromovinyl-wad.

3562 rig/ml. These subjects were between 61 and 77 years old, with estimated creatinine clearance rates that ranged from 38 to 88 ml/min. The highest concentrations of sorivudine were observed in the subject who had the lowest estimated creatinine clearance rate and who had detectable sorivudine through day 17.

There was considerable intersubject variability in the serum concentration of BW. The intervals in days between the cessation of dosing and the day on which serum BVU was undetectable are summa- rized in Table II. BVU was undetectable in the serum of 12 patients by study day 14 (4 days after end of therapy) and in all subjects by day 17. The overall correlation (r) of serum concentrations of sorivudine with those of BW was 0.72 (JJ < 0.001).

DPD Activity in PBMCs. DPD activity in PBMCs was rapidly suppressed after the first dose of sorivudine, and no detectable DPD activity was found in 18 of 19 subjects throughout the period of sorivudine administration (Table III). DPD activity was markedly suppressed in the remaining subject (subject ll), being undetectable on day 2 of sorivudine treatment and remaining at <lo% of the baseline level while BVU was in the circulation. This subject had an extremely high baseline DPD activity of 0.553 nmol/min/mg.

The suppression of DPD activity continued be- yond the period of dosing, with the median intervals between the last sorivudine dose and reappearance of DPD activity in PBMCs being 7 days (range, 0 to 14 days; Table IV). The median of intervals between

568 Tan et al. CLINICAL PHARMACOLOGY &THERAPEUTICS

MAY 1997

Table III. Effect of oral administration of sorivudine on DPD activity in PBMCs

Nominal Days study day after dosing

DPD activity (nmollminlmg protein) n Median Range

Uracil concentration (FOUL) n Mean SD

1 2 5 7

10 14 17 21 24 28 31 35 38

- -

T 4 7

11 14 18 21 2.5 28

19 0.131 0.062-0.553 14 0.178 0.089 19 0.000 0.000-0.000 14 4.897 2.917 16 0.000 0.000-0.044 14 8.216 4.228 17 0.000 0.000-0.000 13 8.448 3.390 18 0.000 0.000-0.032 14 8.180 3.782 18 0.000 0.000-0.049 14 0.990 1.499 18 0.043 0.000-0.340 13 0.321 0.268 17 0.111 0.016-0.240 14 0.241 0.161 16 0.139 0.049-0.226 7 0.205 0.116 8 0.131 0.061-0.202 6 0.161 0.116 7 0.148 0.070-0.360 8 0.134 0.116 4 0.127 0.081-0.203 3 0.152 0.134 4 0.114 0.103-0.152 5 0.205 0.152

DPD, Dihydropyrimidine dehydrogenase; PBMCs, peripheral blood mononuclear cells.

Table IV. Numbers of days when DPD activity reappeared or returned to baseline

Study day Day

after dosing

Reappearance of DPD activity No. of % of

subjects subjects

Return of DPD to baseline No. of % of

subjects subjects

10 0 1 14 4 4 16 6 1 17 7 9 18 8 0 21 11 2 24 14 2 29 19 -

the last sorivudine dose and the return of DPD activity to baseline levels was 11 days (range, 7 to 19 days). Recovery of enzyme activity to baseline had occurred by 14 days after the last dose of sorivudine in 18 of 19 subjects. In the remaining subject (subject 6), in whom the baseline DPD activity was 0.067 nmol/min/mg protein, the DPD activity reappeared 4 days after the last sorivudine dose and the activity remained between 0.012 and 0.016 nmol/min/mg on three occasions between 4 to 11 days after dosing, recovering to 0.049, 0.077, and 0.070 nmol/min/mg on days 13, 19, and 21 after dosing, respectively. In all subjects, the interval between the reappearance of DPD activity and the return of DPD to baseline ranged from 0 to 5 days (Fig. 2).

In all subjects, return of measurable DPD activity occurred only at or after elimination of BVU from serum (Fig. 3).

Circulating uracil concentrations. Circulating uracil concentrations were determined both for use as a

5 - - 21 - -

5 - 47 T 26

0 1 5 11 5 26 11 7 37 - 1 5

functional measurement of DPD activity and to indicate the potential for sustained elevated levels of 5-fluorouracil of toxic or therapeutic relevance. Plasma uracil concentrations for eight subjects and serum uracil concentrations for six subjects were obtained. The mean uracil concentration, which was 0.178 2 0.089 pmol/L before administration of sorivudine, rose rapidly after a single dose of sorivudine, increasing to 4.897 + 2.917 pmol/L on day 2 (Table III). The mean concentration of uracil remained elevated during sorivudine therapy and appeared to plateau at approximately 8.20 pmol/L from day 5 through day 10. By day 14 (4 days after the end of drug therapy), it had dropped sharply to 0.99 pmol/L and appeared to reach baseline within 11 days after the last dose of sorivudine.

The mean uracil and BVU concentrations over time are shown in Fig. 4. The overall correlation (r) of circulating concentrations of uracil and BVU was 0.80 (p = 0.0001). Linear regression analysis indi-

CLINICAL PHARMA COLOGY &THERAPEUTICS VOLUME 61, NUMBER 5 Tan et al. 569

100

0 i 1 51

4 Jll

6

Postdays

Fig. 2. Cumulative percentages of reappearance of dihydropyrimidine dehydrogenase activity (open squares) and return to baseline DPD activity (solid squares) in a total of 19 patients who received oral doses of 40 mg sorivudine once daily for 10 days.

cated a strong relationship between concentrations of uracil and BVU (Y” = 0.77). A significant qua- dratic term (p = 0.0001) in the regression model indicates that the relationship may not be linear; however, review of the data seems to indicate a strictly linear relationship throughout most of the data, with a possible asymptote for uracil at higher BVU concentrations. The intercept in the regression model was not significantly different from zero (p = 0.48). The linear model (ANOVA) showed a significant BVU (‘JJ = 0.0001) and BVU-subject interaction (p = O.OOOl), indicating differences in the relationship between subjects. The partial correlation coefficient (r), adjusting for differences between subjects, was 0.92. The individual subject correla- tions ranged from 0.76 to 1.00, excluding one subject (subject 4) whose correlation was -0.03. This subject may have interrupted his sorivudine therapy.

DISCUSSION In this study concentrations of both sorivudine

and BVU were monitored during dosing (trough level) and after the completion of sorivudine therapy. Consistently elevated sorivudine concentrations were found in four subjects and were attributable to age-related reductions in renal function in three of these subjects. The highest concentration of sorivudine occurred in the subject who had the greatest

degree of renal impairment (estimated creatinine clearance, 38 ml/min). In this subject, BVU was undetectable in the plasma by day 14, whereas sorivudine was detectable until day 17. This observation is consistent with data from single- and multiple-dose studies in elderly subjects and a study in subjects with renal impairment that showed that the pharmacokinetics of sorivudine, but not BVU, were altered with reduced renal function.* Serum concentrations of sorivudine and BVU were unaffected in the subject who had a 6-year history of chronic hepatitis B infection associated with a considerably increased alkaline phosphatase activity.

BVU was eliminated from the circulation within 4 days after the last sorivudine dose in 13 of the subjects and within 7 days of the last dose for the remaining six subjects. The mean elimination half- life value of BVU at sorivudine steady-state previously was determined to be 13.5 +- 5.2 hours in healthy volunteers younger than 50 years old, with

“Olsen SJ, Nicola PS. Oral once-daily multiple-dose and pharmacokinetic study of SQ32,756 in healthy elderly subjects. Protocol No. 32,793-17. Bristol-Myers Squibb Report No. 910045004.

Mangold B, Bogaert J, O’Grady P. Single-dose pharmacokinetics of sorivudine in subjects with normal renal function and with mild, moderate, and severe renal impairment. Protocol No. AI458-060. Bristol-Myers Squibb Report No. 910053101.

570 Tan et al. CLINICAL PHARMACOLOGY &THERAPEUTICS

MAY 1997

- DPD

10 15 20 25 30

Day

Fig. 3. Relationship of mean bromovinyl-uracil (BVU) concentration with median dihydropyrimidine dehydrogenase (DPD) activities in patients who received oral doses of 40 mg sorivudine once daily for 10 days.

serum BVU being undetectable 4 days after the final dose of sorivudine.*

The primary purpose of this study was to define the time course for the recovery of DPD activity after sorivudine therapy and thus to provide infor- mation concerning when subsequent treatment with fluoropyrimidine could be initiated without risk of toxicity. In this study, sorivudine therapy was associated with suppression of DPD activity in all subjects. DPD activity in PBMCs was completely suppressed through the dosing period of sorivudine in 18 of 19 subjects. The other subject had extremely high baseline DPD activity and complete suppression was observed only on day 2 of dosing. This subject may have had sufficient synthetic capacity so that more DPD could be produced than could be bound by BVU, but qualitatively he behaved like all other subjects. These results confirm those of the earlier in vitro study of Desgranges et a1.6

The reappearance of DPD activity in PBMCs seemed to be dependent on the elimination of BVU from the circulation. Two subjects were possible exceptions to this observation. In one subject, DPD activity was not completely suppressed during the

*Stewart MB. Phannacokinetics of SQ32,756 in healthy young and elderly male subjects. Protocol No. 32,793-03. Bristol- Myers Squibb Report No. 910005987.

administration of sorivudine except on day 2, but DPD activity increased after serum BVU became undetectable. In another subject, who may have interrupted sorivudine therapy, DPD activity was present on day 14 at the same time as BVU was present in the circulation. There was considerable variation in the time between the disappearance of BVU from the circulation and the recovery of baseline DPD activity, as well as between the reappearance of initial DPD activity and recovery of baseline DPD activity. This variation may reflect intrinsic differences in enzyme production between individuals.

The return of DPD activity to 0.064 nmol/min/mg, which we have established previously as the limit of activity below which 5-fluorouracil toxicity may occur in patients with genetic deficiencies,r3 may be a more suitable reference point than return to baseline levels. These levels were attained from 7 to 19 days after the last dose of sorivudine. Restoration of DPD activity occurred by 14 days after administration of sorivudine in 18 of 19 subjects and 19 days after administration in the other subject. In this subject, no samples were available between 14 and 18 days after administraiton, and it is likely that recovery of DPD activity to the 95th percentile level in this subject occurred sooner than 19 days after

CLINICAL PHARMA COLOGY & THERAFEUTICS VOLUME 61, NUMBER 5 Tan et al. 571

- Uracil - BVU

6 200

Fig. 4. Relationship of mean uracil concentration with mean BVU concentration in patients who received oral doses of 40 mg sorivudine once daily for 10 days.

administration. The recovery of DPD activity in the three subjects with renal impairment and the single subject with chronic hepatitis B was no different from that in the other subjects.

In this study, suppression of DPD activity was associated with a considerable increase in circulating uracil concentrations in the 14 subjects in whom it was measured. This observation is not surprising because DPD is the initial step in the catabolism of both exogenous and endogenous pyrimidine bases. Elevations of uracil concentration have been observed in genetic DPD deficiency,9,‘0721 and increased levels of uracil seem to be a good indication of the degree of enzyme deficiency.22 Complete suppression of DPD activity after exposure to 5-propynyluracil and 5-ethynyluracil (which, like BVU, are irreversible inhibitors of DPD) has been associated with an increase in plasma uracil concentration in healthy volunteers and patients with cancer.23,24 In this study, there was a positive correlation between uracil and BVU concentrations (Y” = 0.77). The partial correlation coefficient, adjusting for differences across subjects, was 0.92. This result suggests that circulating uracil may be a marker of total body DPD activity and that the increases in uracil levels may reflect the degree of inhibition of

total body DPD after oral administration of sorivudine.

The uracil concentration had decreased considerably by day 4 after the last dose of sorivudine in all subjects in whom it was measured. The circulating uracil concentration returned to baseline much sooner than DPD activity in PBMCs, which suggests that there is an upturn in total body DPD activity before DPD activity is detectable in PBMCs. It has been shown previously in rats given 5-ethynyluracil that plasma uracil concentrations began to fall within 24 hours after dosing, at a time when approx- imatelv 70% of the heoatic DPD remained inacti-

Based on the data from the subjects enrolled in this study, a 4-week period after the last dose of sorivudine should be more than sufficient to permit adequate recovery of DPD activity before initiation or resumption of administration of fluorinated pyrimidine. In the subjects in this study, the last detectable concentrations of BVU were observed 4 days after the end of therapy, which would leave a 24-day period for recovery of DPD activity. In all but one subject, DPD activity had returned to baseline 14 days after the completion of sorivudine treatment and by 19 days, and possibly sooner, in all

572 Yun et al.

subjects. This time course to recovery of DPD activity in PBMCs, together with the much earlier reappearance of DPD activity as indicated by the decrease in uracil concentrations, suggests that it is probably safe to commence therapy with a fluorinated pyrimidine, such as Sfluorouracil, flucytosine, or floxuridine 14 days after the completion of sorivudine therapy. However, a longer period is preferable to provide a greater margin of safety for those subjects who may eliminate BVU more slowly or those who take longer to regenerate DPD. In this regard, subjects with hepatic impairment have been described as having a mean BVU half-life of 32 hours (range, 17.8 to 45.5 hours) compared with 16 hours (range, 15.5 to 26.9 hours) in age- and gender- matched normal subjects.* This twofold increase in elimination time suggests that BVU could still be in circulation 8 days after the end of sorivudine administration but would ensure sufficient time for the recovery of DPD activity within 4 weeks. These data support the recommendation that therapy with a fluorinated pyrimidine can be begun 4 weeks after the completion of sorivudine treatment without risk of toxicity.

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The effect of sorivudine on dihydropyrimidine dehydrogenase activity in patients with acute herpes zoster*