1 Department of Biopharmaceutical Sciences, University of California, San Francisco, CA and 2 Bone Care International, Inc., Middleton, WI, USA
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Abstract |
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Methods. 1(OH)D2 was administered in a range of single and multiple doses to volunteers with and without normal renal and/or hepatic function. Subsequent serial blood samples were assayed by HPLC/radioimmunoassay for the metabolite 1,25-dihydroxyvitamin D2 [1,25(OH)2D2], the major active species.
Results. Bioavailability of 1,25(OH)2D2 from a single 5 µg 1(OH)D2 oral-capsule dose was estimated to be normally
42% of that from a 5 µg intravenous injection. Steady-state serum concentrations of 1,25(OH)2D2 were attainable within 8 day, and fluctuated
2.5-fold from peak to trough when oral 1
(OH)D2 doses were taken every second day, and the terminal half-life was 34±14 h. Mean steady-state serum concentrations rose less than proportionally (from 20 to 45 pg/ml) on increasing oral 1
(OH)D2 doses from 5 to 15 µg every 48 h. Renal patients showed 39±37% increase in serum 1,25(OH)2D2 concentration during 34 h haemodialysis sessions, but no other difference in steady-state pharmacokinetics was found between these or hepatically impaired patients and normal subjects.
Conclusions. Given the sensitivity limits of current assays, the pharmacokinetics of this and other vitamin-D compounds is best elucidated from steady-state studies. The pharmacokinetics of 1,25(OH)2D2 from 1(OH)D2 doses appears to be similar to that of 1,25(OH)2D3 from 1
(OH)D3 doses, albeit D3 data have to date largely derived from single-dose studies. Deviation of 1,25(OH)2D2 pharmacokinetics from linearity appears to be marginal enough to be clinically manageable with adequate precaution.
Keywords: doxercalciferol; hepatic disease; pharmacokinetics; renal disease; steady state; vitamin D2
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Introduction |
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Alphacalcidol [1-hydroxyvitamin D3 or 1
(OH)D3] has also been used for secondary hyperparathyroidism and for osteoporosis, primarily in Europe and Japan. Because alphacalcidol is hydroxylated at carbon 25 only upon reaching the liver [3], it was hoped that oral doses would avoid dihydroxylated vitamin D, before its own absorption, exerting a direct and local effect that promotes calcium absorption from the gastrointestinal tract [4]. However, the extent to which treatment with alphacalcidol, either by oral or i.v. dosage, does avoid the hypercalcaemia associated with calcitriol is still the subject of scientific debate [5,6].
Based on similar rationale, doxercalciferol, the D2 analogue of alphacalcidol, was also developed to effectively reduce iPTH without causing frequent hypercalcaemia or hyperphosphataemia [7] and it is now in wide clinical use to treat secondary hyperparathyroidism. To date, however, little detailed pharmacokinetic information has been available on doxercalciferol and clinically relevant pharmacokinetics is not straightforward to predict, given that the active species is not doxercalciferol per se, but its metabolite, 1,25-dihydroxyvitamin D2 [1,25(OH)2D2]. In order to optimize use of doxercalciferol, therefore, it is important to characterize its pharmacokinetics, not only in normal subjects but also in the populations to be treated (e.g. renal disease and osteopenia patients) and, further, in hepatically impaired patients, in whom both the formation and elimination of 1,25(OH)2D2 might be compromised.
This report covers four studies, each describing the pharmacokinetics of 1,25(OH)2D2 arising from doses of the prodrug 1(OH)D2. Study A was designed to investigate the bioavailability and pharmacokinetics of 1,25(OH)2D2 from single oral and i.v. doses of 1
(OH)D2 to post-menopausal women with osteopenia and the potential dose-dependence of such pharmacokinetics. Using multiple-dose oral regimens, study B investigated steady-state pharmacokinetics and its potential dose dependence in normal young men and women, study C the impact of renal disease and haemodialysis and study D the impact of varying degrees of hepatic disease.
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Subjects and methods |
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Study A
An oral placebo dose in 22 osteopenic women with normal kidney and liver function was followed by a three-treatment [5 µg i.v. and 2 and 5 µg oral doses of 1(OH)D2] crossover design, with 1 week intervening between each of the four doses. The six possible sequences of the three drug treatments were each randomly assigned to four patients but two patients withdrew for personal reasons (one on the 5 µg oral first and i.v. last, one on the 5 µg oral first and 2 µg oral last). All subjects were Caucasian, 5876 years old, 5295 kg, 528 years post-menopausal, with a 0.741.05 g/cm2 vertebral L2L3 mineral bone density as measured anteroposterior by dual-energy X-ray absorptiometry. Each had had low vitamin-D2 intake for the preceding year. Doses were given
1 h or less before breakfast and blood samples were taken just before each dose and 2, 4, 6, 8, 10, 12, 14, 16, 20, 28 and 48 h afterwards.
Study B
This was a two-treatment crossover study in which, for each treatment [5 or 15 µg oral 1(OH)D2 doses], 24 normal subjects (12 per gender) each received a dose every 48 h for 5 doses. The two treatments were separated by at least 14 days, with six women and six men chosen randomly to receive one of the treatments first and the remaining subjects the other first. All subjects were in normal health according to physical examination and standard blood screens. The women (four African-American, eight Caucasian) were 2035 years old and 39109 kg; the men (three African-American, nine Caucasian) were also 2035 years, but 59118 kg. Doses were given
1 h or less before breakfast. For each regimen, blood samples were taken on three occasions at least 24 h apart during the week preceding the first dose, just before each of the remaining doses and 2, 4, 6, 8, 12, 16, 24, 36, 48, 72, 96 and 120 h after the final dose.
Study C
Fourteen haemodialysis patients each received a 10 µg oral 1(OH)D2 dose every 48 h for five doses, but 48 h following the 5th dose, after three patients had developed hypercalcaemia and were withdrawn, the dose for the remaining 11 patients was lowered to 5 µg every 48 h for a further five doses. These patients (four women, 4573 years, 5283 kg; seven men, 2969 years, 6182 kg; all Caucasian other than one Hispanic woman and one African-American man) had a serum creatinine concentration of
6 mg/dl (9.4±3.0 mg/dl) and had been on haemodialysis three times per week for >1 month. During the study, dialysis was at two 2 day and one 3 day intervals per week. Doses were given immediately after dialysis and at the same time of day on dosing days without dialysis. Each regimen was commenced such that its last dose was taken after a dialysis preceded by the two 2 day interdialysis intervals. Patients underwent 1 week free of vitamin-D supplementation prior to receiving the first study dose. Blood samples were taken immediately before that dose and before and after dialysis on the 8th day of dosing (the first dialysis after a 3 day interdialysis interval), just before the 8th, 9th and 10th (final) doses and 2, 4, 6, 8, 10, 12, 16, 24, 36, 48, 72, 96, 120, 144, 168 and 240 h after the 10th dose.
Study D
Thirteen patients with hepatic impairment and four normal subjects received a 10 µg oral 1(OH)D2 dose every 48 h for eight doses. The patients were subcategorized as having mild hepatic impairment if they scored 56 points, moderate impairment 79 points and severe impairment 1015 points, by the Pugh modification of the ChildTurcotte hepatic-impairment criterion [8]. Accordingly, there were three women (4065 years, 6482 kg) and five men (4370 years, 67100 kg) with mild hepatic disease, two men (4960 years, 8998 kg) with moderate disease and one woman (41 years, 64 kg) and two men (4058 years, 89105 kg) with severe disease. Normal subjects had passed a physical examination and standard blood screens and included two women (4851 years, 7187 kg) and two men (4148 years, 6875 kg). All subjects were Caucasian other than one Asian woman with mild hepatic disease. Doses were given
1 h or less before breakfast. Blood samples were collected during a study-screening visit to the clinic between 1 and 8 days before the first dose of the study, subjects having avoided vitamin-D supplements for the prior month. Blood samples were also obtained just before the 5th, 6th, 7th and 8th (final) doses and 3, 6, 8, 10, 12, 16, 24, 36, 48, 72, 96, 120 and 168 h after the 8th dose.
Assay
Plasma and serum samples, after addition of internal standard [3H-1,25(OH)2D3], were extracted with acetonitrile that, after a phosphate-buffer wash, was applied to a C18OH solid-phase-extraction cartridge (BondElut; Varian, CA, USA). After cartridge rinses with a series of solvents, a hexane/isopropanol eluate was collected, evaporated under nitrogen and reconstituted to 200 µl. This was injected onto a Zorbax-SIL 4.6x25 cm HPLC column (MAC-MOD, PA, USA) with pre-column (Adsorbosphere Silica, 5 µ; Alltech, IL, USA), eluted at 2 ml/min with hexane/methylene chloride/isopropanol (47.9%/47.9%/4.2%). Eluate fractions were evaporated and that containing internal standard was reconstituted in 150 µl ethanol for liquid scintillation counting to determine extraction/elution recovery. The fraction containing 1,25(OH)2D2 was reconstituted in 50 µl ethanol and subjected to a radioreceptor assay (study A) or radioimmunoassay (studies BD) based upon commercially available 1,25-dihydroxyvitamin-D assay kits (DiaSorin, MN, USA).
The assay limit was 1 pg/ml (
2 pg/ml in study C). Assay bias at each of several control concentrations was estimated as the percentage deviation of the mean assay result (corrected for innate concentration in blank plasma or serum) from the spiked concentration in six separately spiked replicate control samples (not standard-curve calibrators) at that concentration. In plasma, averaged across three assay runs, the bias at 5, 10, 20 and 40 pg/ml, respectively, was 15±5%, 8±10%, 6±5% and 8±2% high, with coefficients of variation amongst the six replicate samples of 17±6%, 14±8%, 10±3% and 12±10% at these four respective concentrations. In serum, averaged across five assay runs, the bias at 5 (one assay run only), 10, 30, 100 and 200 (two runs only) pg/ml was 16%, -12±4%, -13±4%, -13±4% and 4±8%, respectively, with coefficients of variation of 14%, 9±2%, 6±1%, 9±3% and 7±3%.
Pharmacokinetic analysis
Each subject's innate steady-state baseline concentration of dose-unrelated 1,25(OH)2D2 was estimated in study A as AUC(048)/48 h from the placebo dose (AUC is described below). In studies C and D, the concentration in the serum sample taken before commencing the first regimen was used and in study B the mean of the three such concentrations. In study C, six patients had a baseline 1,25(OH)2D2 below assay sensitivity, so their baseline concentration was taken as half the limit of assay sensitivity on the day of assay.
Half-life was estimated as ln(2)/z where
z, the terminal rate-constant, was minus the terminal slope of the plasma or serum concentration (C) vs time (t) curve (semilogarithmic) after each treatment in each subject. This slope was estimated from an unweighted least-squares linear fit utilizing the last three or (if, in studies BD, it gave a fit with a higher correlation coefficient squared) four points on the curve before they declined below double the baseline concentration in that subject. Concentrations were entered into the fit after subtracting that baseline concentration.
AUC (the area under the plasma or serum concentrationtime curve) was calculated as AUC extrapolated from single doses and as AUC interdose from multiple doses. Firstly, AUC unextrapolated was estimated, from the time of the (last) dose to the time of the assayable sample taken closest to 48 h, using the trapezoidal and logarithmic-trapezoidal equations, respectively, for periods of increasing (or stationary) and decreasing concentrations. To obtain AUC extrapolated, AUC unextrapolated from single doses was extrapolated to infinite time by adding (Clast Cbaseline)/z and to obtain AUC interdose, AUC unextrapolated from multiple doses was adjusted for interdose intervals not exactly as scheduled by adding [(Csched48 Cactual48)x(48.0 tactual48)]/[ln(Csched48/Cactual48)] where Cactual48 is the concentration in the sample scheduled for 48 h but really collected at tactual48 and where Csched48=(Cactual48 Cbaseline)xe-
Zx(48tactual48)+Cbaseline. (Throughout studies BD, this adjustment for inexact interdose intervals accounted for <1.3% of AUC.) The resultant area yielded AUC extrapolated and AUC interdose after it was corrected for innate 1,25(OH)2D2 not dose-generated, by subtracting AUC(0 48 h) for the placebo dose in study A and the baseline concentration multiplied by 48 h in the other studies.
Bioavailability of test relative to reference treatment was estimated as (AUCtest/AUCreference)x(Dosereference/Dosetest). Peak plasma or serum concentration was taken as the concentration in the sample with the highest measured concentration minus the baseline in that subject and peak time was the time of that sample. For studies BD, the trough concentration and time of trough were estimated analogously from the sample with the least concentration within 48 h after the last dose of each treatment. Where, for some hepatically impaired subjects, the baseline concentration estimate appeared greater than the least observed concentration, the trough concentration was taken as zero. Peak-to-trough swing refers to the peak minus trough concentrations.
Statistical analysis
Data are shown as means±SD. Comparisons were made by analysis of variance (ANOVA) based on the type III sums of squares using SAS software (SAS Institute, Cary, NC, USA). All parameters were corrected for dose size (and, in study B, for the subject's body weight) before logarithmic (natural) transformation and then analysis, except that peak and trough times were analysed uncorrected and untransformed. For studies A and B, ANOVA analysed for treatment and period as within-subject factors and sequence and (in study B) gender as between-subjects factors. For studies C and D, one-factor (subject group) ANOVAs were performed, with, for study D, a test for linear trend according to severity of diseasefrom normal, to mildly impaired, to moderately, to severely. Additionally, for studies BD, pre-dose concentrations and the concentration 48 h after the last dose were all compared by ANOVA analysing for subject and day-of-trough as factors and for linear trend in the days of trough. Multi-sample ANOVAs, if found significant, were followed by a Tukey pairwise-comparison procedure. For studies A and B, bioequivalence of treatments was also investigated by the Schuirmann two one-sided (=0.05) confidence intervals (CIs) test that yielded a 90% CI for the ratio, between test and standard treatments, of each parameter processed by ANOVA as its logarithmic transform [9].
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Results |
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Study B
Baseline concentrations were 2.72±1.18 pg/ml. No significant difference or linear trend with study day was distinguishable between trough serum concentrations 2 days prior to, immediately before and one normal interdose interval (48 h) after the last dose, indicating attainment of steady state. Figure 2 shows concentrations during an interdose interval at steady state and Figure 3
(logarithmic) concentrations for a full 120 h after the last dose. Half-life was estimated to be 34±14 h.
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Study C
Baseline concentrations were 2.18±2.13 pg/ml. On day 8, post-dialysis concentrations were found to be greater than those pre-dialysis by 39±37% (P=0.003), doubling in two of the patients (from 63.4 to 126 pg/ml and from 35.3 to 75.0 pg/ml), but none of the steady-state pharmacokinetic parameters for renal patients was found different from that for normal subjects on the same dose in study B (Table 3).
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Study D
Baseline concentrations were, respectively, 6.70±3.02, 8.15±2.19, 5.77±3.61 and 7.33±2.14 pg/ml in the patients with mild, moderate, severe and no hepatic impairment. No significant difference or linear trend with study day was distinguishable (both for all patients and for each disease-state group) between trough concentrations 2 days prior to, immediately before and one normal interdose interval (48 h) after the last dose, indicating attainment of steady state. No significant differences or trends in the steady-state parameters measured were identified statistically between the varying degrees of hepatic disease (Table 4). Each of the moderately impaired patients, however, showed an AUC that appeared remarkably low (682±504, 738±646 and 711±502 vs 85±87 hxpg/ml, respectively, for mild, severe and no impairment vs moderate impairment), but there were only two moderately impaired patients and their AUCs were so marginally (47% and 5%) above baseline as to be difficult to measure precisely. For AUC, ANOVA found P=0.067.
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Discussion |
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This is a limitation that study A shares with virtually all previously published reports on vitamin-D pharmacokinetics, but which studies BD, described here, largely escape due to their steady-state design. For steady-state concentrations, the AUC that relates to bioavailability and clearance is the interdose AUC, whose estimation does not require extrapolation and is, thus, not sensitive to terminal rate-constant estimates. In any case, estimation of even the terminal rate-constant itself is also assisted by the larger concentrations usual after steady-state doses than after a single dose. Furthermore, there is more direct assurance that concentrations actually observed at steady state, particularly if they happen to arise from non-linear mechanisms (as found here), will reflect concentrations eventuating during multi-dose clinical use than there is from concentrations projected from single-dose studies.
Achieving steady state
In order to gain a more reliable pharmacokinetic description in the face of the sensitivity limitations of current assays, a multiple-dosing approach was implemented in studies subsequent to study A. A comparison of trough concentrations in study B after 6, 8 and 10 days had elapsed, indicates that steady-state conditions can be reasonably approximated with 8 days of dosing, consistent with the half-life estimates observed. Since it takes 3.3 half-lives to reach 90% of steady-state concentrations, only subjects with a half-life >58 h would not have reached 90% of steady-state levels by 8 days. In study B, of the 42 half-lives that could be estimated from terminal slopes fit with correlation coefficients >0.75 (32±11 and 37±16 h after the 5 and 15 µg regimens, respectively; n=21), only one had an estimate (95 h) >58 h.
Terminal half-life
Half-lives were usually substantially longer than 24 h (34±14 h in normal young adults in study B). Since, additionally in study A, the time of peak 1,25(OH)2D2 concentrations after i.v. 1(OH)D2 injections (8.0±5.9 h) was far from instantaneous (and not significantly different after oral doses; 11.1±5.0 h), it is possible that the terminal half-life for 1,25(OH)2D2 might be relatively long because it reflects a rate-limiting formation of this species from the 1
(OH)D2 dosed entity, rather than the elimination of 1,25(OH)2D2 itself (the terminal phase reflects the slowest process [11]). Indeed, this may be analogous to the case with the vitamin-D3 compounds, since both the terminal half-life and peak time of 1,25(OH)2D3 after oral dosing with 1,25(OH)2D3 have been reported to be only
50% of those of 1,25(OH)2D3 after oral dosing with 1
(OH)D3 (half-life: 13±2 vs 36±11 h [12], 28±9 vs 47±10 h [13]; peak time: 2.3±1.0 vs 6.5±3.6 h [13], 4.3±1.3 vs 11±3 h [14]). (The D3 half-life estimates were, however, reported from studies with a limited number of blood samples collected and/or with assayable samples only until 24 h after dosing.) Normally, 25-hydroxylation of dietary vitamin D2 and D3 is observed to occur largely before 1-hydroxylation [1,15]. Therapies employing alphacalcidol and doxercalciferol, which are already hydroxylated at the 1-position, reverse the innate sequence and the uncovering of a normally bypassed rate-limiting conversion might be the result. Clinically, this might prove more of an advantage than a disadvantage (as discussed below).
Deviation from pharmacokinetic linearity
Study B shows that serum concentrations rise less than proportionally on raising the dose from 5 µg every 48 h to 15 µg every 48 h. AUC during the 15 µg regimen was only 85.7±33.5%, peak concentration only 85.7±35.4% and the peak-trough concentration difference only 83.9±34.4% of those that would have been predicted from the 5 µg regimen, had pharmacokinetics been linear (and after taking into account endogenous background concentrations). Peak concentration behaved similarly in study A. The similar percentage by which AUC, peak concentration and the peak-to-trough concentration swing deviate from linear expectations is consistent with saturation in one or more process responsible for metabolic conversion of 1(OH)D2 to 1,25(OH)2D2, but other mechanisms are also possible (e.g. saturable gastrointestinal transport, saturable plasma-protein binding and saturable renal reabsorption). This marginal non-linearity in 1,25(OH)2D2 formation and/or disposition kinetics might explain in part why the metabolite 1,24-dihydroxyvitamin D2, a vitamin D of interest because of its reportedly low calcaemic activity [16], appears at only
10% or less of the concentrations of 1,25(OH)2D2 after standard doses of vitamin D2, but in increasing proportions after higher doses of vitamin D2 [17,18].
Comparison of vitamin-D2 and -D3 pharmacokinetics
The terminal half-life of 1,25(OH)2D2 was about 36 h in normal subjects taking multiple oral 1(OH)D2 doses (study B). By comparison, 1,25(OH)2D3 terminal half-lives of, respectively, 36±11 and 48±4 h were observed by Kimura [12] and Brandi [13] and their colleagues in haemodialysis patients taking 1
(OH)D3 orally and in normal subjects receiving 1
(OH)D3 either orally or intravenously. Additionally, Joffe et al. [19] reported that the half-life after i.v. 1
(OH)D3 doses in peritoneal-dialysis patients appeared to be several days on average, although in the eight subjects studied the estimate covered a 25-fold range. The accuracy of estimation of 1,25(OH)2D3 half-lives as large as 36 h or more is, however, uncertain in all three of the D3 studies as each is hampered either by no samples being collected after 24 h following a single dose or by such samples not being significantly above innate baseline levels (peak times were typically 7 h or substantially longer). Further, this uncertainty is transmitted into estimates of extrapolated AUC, of which 50% or substantially more might reside in the extrapolation after 24 h [19], but with such extrapolation relying on the uncertain half-life estimate.
Using data up until 24 h, however, Joffe et al. [19] found a 1,25(OH)2D3 AUC of 1316±237 hxpg/ml after a single 5.6±1.7 µg (80 ng/kg) i.v. 1(OH)D3 dose in five men and three women on peritoneal dialysis and a mean serum concentration of 62±13 pg/ml at 4 h, the highest after this dose. This is comparable with study A above in which the 1,25(OH)2D2 AUC to 28 h (the sample nearest 24 h) was 1130±550 hxpg/ml after a single 5 µg (
70 ng/kg) i.v. 1
(OH)D2 dose in 22 osteopenic but otherwise normal women and the mean concentration at 4 h was 53±32 pg/ml, also the highest after this dose. Brandi et al. [13] also showed a comparable 1,25(OH)2D3 AUC to 72 h (approximating AUC to 24 h if concentrations after 24 h were essentially background as reported) of 1366±757 hxpg/ml and a mean plasma concentration of
50 pg/ml 4 h after an i.v. dose of 4 µg (
70 ng/kg) to six normal women. Thus, the data available suggest serum concentrations of a magnitude similar between 1,25(OH)2D2 and 1,25(OH)2D3 at least in the first 24 h after administering similar doses of their respective 1
-monohydroxylated prodrugs.
In study A, less than half of orally dosed 1(OH)D2 appeared to reach the systemic circulation as 1,25(OH)2D2. By comparison, Joffe et al. [19] found that oral doses of 1
(OH)D3 given to peritoneal-dialysis patients gave rise to a mean 1,25(OH)2D3 unextrapolated AUC 65% of that from i.v. 1
(OH)D3 doses of the same strength. The data of Brandi et al. [13] show this figure to be 80% in normal subjects and, further, that the unextrapolated AUC from either oral or i.v. 1
(OH)D3 doses was only
40% of that from 1,25(OH)2D3 given as such intravenously. This would indicate that probably as much, or more, of the active species is lost in the conversion of 1
(OH)D3 to 1,25(OH)2D3 than in oral absorption of the former. Whether this proves to be so depends upon whether the relationships observed between unextrapolated AUCs reflect those that exist between extrapolated AUCs, which are a much closer index of bioavailability, but which could not be confidently estimated from single-dose data.
Renally impaired patients on haemodialysis
Haemodialysed renal patients receiving 1(OH)D2, were not found different from normal subjects in their steady state 1,25(OH)2D2 pharmacokinetic parameters, but had substantial increases in serum concentration during a dialysis session. As the previous dose was 3 days beforehand, this increase and its rapidity is indicative of a decrease in volume of distribution. Yet a loss of so much fluid during dialysis as to account directly for a doubling of concentration in two of the patients is unlikely [20]. In renal disease, increased volumes of distribution of drugs with normally significant plasma-protein binding [1,25(OH)2D2 >99%; Bone Care International, WI, USA, unpublished data] are not unusual [10] and are largely the result of competition for binding proteins from endogenous substances normally, but no longer, cleared by a healthy kidney. Such substances tend to be also dialysable and, if they are lost during a dialysis session, drug binding can increase, volumes of distribution decrease and drug concentrations increase. Protein-binding studies would clarify this question and also whether the lack of difference apparent in total serum concentrations between renal patients (after dialysis) and normal subjects is true also for concentrations of unbound drug, since the latter are usually more directly predictive of effect. A lack of significant impact of renal disease on the pharmacokinetics of unbound 1,25(OH)2D2 from doses of 1
(OH)D2 would, however, be unsurprising, given that 25-hydroxylation is mediated by hepatic enzymes, not renal [3], and assuming that elimination of the large hydrophobic 1,25(OH)2D2 molecule is also predominantly hepatic (and is relatively low extraction).
Hepatically impaired patients
The metabolic pattern described above predicts a direct effect by hepatic disease on the pharmacokinetics of 1,25(OH)2D2 arising from 1(OH)D2 dosing. Yet, in the face of the within-group variability seen even in normal subjects (10-fold range in AUC from the 5 µg study B regimen, 1842118 hxpg/ml; 957±533 hxpg/ml), it seems that more subjects than the two to eight per subgroup in study D would be needed to answer this question conclusively. Nevertheless, it is noteworthy that not only normal subjects and mildly impaired patients, but also severely impaired patients, yielded apparently very similar mean steady-state concentrations (14.8±10.5, 14.2±10.5 and 15.4±13.5 pg/ml) in study D, while it was the two moderately impaired patients who yielded remarkably low concentrations (3.1 and 0.5 pg/ml). This apparent divergence in the moderate but not severe group might be chance, but might, however, instead arise because 1,25(OH)2D2 concentrations reflect an interplay between the effects of the level of hepatic function not only on elimination of 1,25(OH)2D2, but also on its production from 1
(OH)D2. Any influence of hepatic disease on 1,25(OH)2D2 pharmacokinetics might, therefore, not be simple either to ascertain or to characterize and might depend on a more specific diagnosis than just hepatic disease.
Clinical ramifications
Steady-state concentrations of 1,25(OH)2D2 appear to be attainable in most normal and renal-failure patients within about 1 week or a little longer after commencing dosage with 1(OH)D2, which, given the use of this agent, should not prove clinically burdensome. A half-life of >1 day allows dosing with this agent as infrequently as every day or even three times per week, with more modest interdose fluctuations in concentration than achieved with vitamin-D treatments exhibiting less half-life. While there is some non-linearity in the pharmacokinetics of 1,25(OH)2D2, causing concentrations to rise less than proportionally with doses of 1
(OH)D2, an overestimate of only 15% in mean steady-state concentrations when tripling the dose from 5 µg every 48 h to 15 µg every 48 h does appear to be of low enough magnitude to be clinically manageable with adequate precaution (particularly since this is a drug where the current recommendation is continual monitoring and dose titration). Moreover, this deviation from linear pharmacokinetics does not itself appear to require separate dosing/monitoring considerations for men and women to any extent that was detectable. It is not known whether analogous non-linearity arises from alphacalcidol [1
(OH)D3] dosage also.
There appears to be no difference in 1,25(OH)2D2 pharmacokinetics in haemodialysis patients which would dictate a dosing schedule different from that of patients with normal renal function. However, such an assessment would be better based on measurements of unbound concentration in plasma, as it would be for hepatically impaired patients also. Pending such studies, it is prudent to undertake careful and periodic evaluation, on an individual-patient basis, of the dosing requirements of patients with impaired renal or hepatic functionality, as is stated in the manufacturer's labelling instructions for this new agent.
Conflict of interest statement. R.A.U. and C.W.B. are paid consultants of Bone Care International Inc., the developer of doxercalciferol; J.C.K. and L.W.L. are employees and C.W.B. was an employee of Bone Care International. C.W.B. holds stock in Bone Care International.
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Acknowledgments |
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Notes |
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References |
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