Pharmacokinetics of 1,25(OH)2D3 and 1{alpha}(OH)D3 in normal and uraemic men

Lisbet Brandi, Martin Egfjord and Klaus Olgaard

Nephrological Department P, Rigshospitalet, University of Copenhagen, Denmark



   Abstract
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 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
Background. The therapeutic equivalence of 1,25(OH)2D3 and 1{alpha}(OH)D3 on the suppression of PTH synthesis and secretion has not clearly been established. The aim of the present study was to evaluate the pharmacokinetics of 1,25(OH)2D3 and 1{alpha}(OH)D3 after oral and i.v. administration in healthy volunteers and uraemic patients.

Methods. Six healthy volunteers and 12 uraemic patients were included in the study. With an interval of 2 weeks, 4 µg of 1,25(OH)2D3 i.v., 4 µg of 1,25(OH)2D3 orally, 4 µg of 1{alpha}(OH)D3 i.v. and 4 µg of 1{alpha}(OH)D3 orally were administered. Blood samples for analysis of plasma-Ca2+, plasma-1,25(OH)2D3, and plasma-PTH were drawn at time 0, 0.25, 0.5, 1, 2, 4, 6, 9, 12, 24, 48, and 72 h. The healthy volunteers were studied in all four protocols and the uraemic patients in either the 1{alpha}(OH)D3 (n=6) or the 1,25(OH)2D3 (n=6) protocol.

Results. After oral administration of 1,25(OH)2D3 the bioavailability of 1,25(OH)2D3 was 70.6±5.8/72.2±4.8% in healthy volunteers/uraemic patients (n.s.). After i.v. administration the volume of distribution of 1,25(OH)2D3 was similar, 0.49±0.14 vs 0.27±0.06 l/kg in healthy volunteers vs uraemic patients (n.s.), while the metabolic clearance rate of 1,25(OH)2D3 was 57% lower in the uraemic patients, 23.5±4.34 vs 10.1±1.35 ml/min in healthy volunteers vs uraemic patients, respectively (P<0.03). The bioavailability of 1,25(OH)2D3 after i.v. administration of 1{alpha}(OH)D3 was 42.4±11.0/42.0±2.0% in healthy volunteers/uraemic patients (n.s.); and after oral administration of 1{alpha}(OH)D3 42.0±2.0/29.8±3.1% in healthy volunteers/uraemic patients (n.s.). A small, but significant increase in plasma-Ca2+ was seen after administration of 1,25(OH)2D3 to the uraemic patients, while no increase was seen after administration of 1{alpha}(OH)D3. PTH levels were significantly suppressed in the healthy volunteers 24 h after administration of 4 µg of 1,25(OH)2D3 i.v., 4 µg of 1,25(OH)2D3 orally, and 4 µg of 1{alpha}(OH)D3 orally by 35±7, 30±8, and 35±4%, respectively (all P<0.03). In the uraemic patients, PTH levels were significantly suppressed after administration of 4 µg of 1,25(OH)2D3 i.v., 4 µg of 1,25(OH)2D3 orally, and 4 µg of 1{alpha}(OH)D3 i.v. by 30±10, 45±7, and 40±7%, respectively (all P<0.04). The effect was transitory in the healthy volunteers and lasted for at least 72 h in the uraemic patients.

Conclusion. The present study found a 57% lower metabolic clearance rate of 1,25(OH)2D3 in uraemic patients, as compared with that of healthy volunteers (P<0.03). The bioavailability of 1,25(OH)2D3 following administration of 1{alpha}(OH)D3 i.v. and orally in both healthy volunteers and uraemic patients was markedly lower than after administration of oral 1,25(OH)2D3 (P<0.03). In spite of lower plasma-1,25(OH)2D3 levels after administration of 1{alpha}(OH)D3, no significant difference was observed on the suppressive effect of 4 µg i.v. of either 1,25(OH)2D3 or 1{alpha}(OH)D3 on the plasma-PTH levels in the uraemic patients. This might suggest the existence of an effect of 1{alpha}(OH)D3 on the parathyroid glands which is independent of the plasma-1,25(OH)2D3 levels, that are achieved after oral or i.v. administration of 1{alpha}(OH)D3.

Keywords: calcitriol; 1{alpha}-hydroxycholecalciferol; hyperparathyroidism secondary; parathyroid hormone; pharmacokinetic; uraemia



   Introduction
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 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
The natural hormone 1,25(OH)2D3 and the synthetic vitamin D analogue 1{alpha}(OH)D3 have been used extensively for prophylaxis and treatment of secondary hyperparathyroidism (sec. HPT) in chronic uraemia [1,2]. Both vitamin D metabolites have a significantly suppressive effect on plasma-PTH synthesis in sec. HPT [1,2]. The therapeutic equivalence between 1,25(OH)2D3 and 1{alpha}(OH)D3 is, however, not clear. Both equal [3] and half potency of 1{alpha}(OH)D3 [4,5], compared with that of 1,25(OH)2D3 on the suppression of PTH have been reported in chronically uraemic patients. Similarly, only sporadic information exists on the pharmacokinetic differences after i.v. and oral administration of the two vitamin D metabolites, especially in chronically uraemic patients. Thus, after oral administration of 1{alpha}(OH)D3 a significantly smaller peak concentration (Cmax) and a smaller area under the curve (AUC) of 1,25(OH)2D3 have been reported than after oral administration of similar doses of 1,25(OH)2D3 [57]. It is well known that the vitamin D analogue, 1{alpha}(OH)D3, is hydroxylated to 1,25(OH)2D3 by the liver. Increasing peak concentrations of 1,25(OH)2D3 were found following increasing doses of i.v. 1{alpha}(OH)D3 [8], but the levels of 1,25(OH)2D3 achieved were still much lower, than those found in studies on i.v. administration of even smaller doses of 1,25(OH)2D3 [9,10]. The transitory high peak concentration of 1,25(OH)2D3 is suggested to be of importance in the execution of the direct, non-calcaemic suppressive effect on the PTH gene transcription [9]. As the clinical effect of 1{alpha}(OH)D3 and 1,25(OH)2D3 on PTH suppression in uraemia seems to be of the same magnitude [1,2], the effect of 1{alpha}(OH)D3 might not only be explained as secondary to the peak concentration of 1,25(OH)2D3.

The aim of the present study therefore was (i) to evaluate the pharmacokinetics of 1,25(OH)2D3 and 1{alpha}(OH)D3 in response to i.v. and oral administration in both healthy volunteers and uraemic patients and (ii) at the same time to measure the effects of a single dose—the same as above—of the two vitamin D analogues on the plasma-PTH and plasma-Ca2+ levels in order to examine, whether possible pharmacokinetic differences would result in different biological responses.



   Subjects and methods
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 Subjects and methods
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Six healthy volunteers and 12 uraemic patients on chronic dialysis were included. Because of the need to obtain 10 ml of blood for each analysis of 1,25(OH)2D3 the uraemic patients had to be divided into two groups (1,25(OH)2D3 and 1{alpha}(OH)D3 group). In the normal control group all were females. Three males and three females were in the 1,25(OH)2D3 group, while two males and four females were in the 1{alpha}(OH)D3 group. The residual endogenous creatinine clearance ranged from 0 to 3 ml/min. In the 1,25(OH)2D3 group four patients were treated by continous ambulatory peritoneal dialysis (CAPD) and two patients by haemodialysis. In the 1{alpha}(OH)D3 group three patients were treated by CAPD and three patients by haemodialysis. The CAPD patients performed four bag exchanges daily. On the study day no bag exchanges were performed 2 h before and 8 h after administration of the vitamin D analogue. The haemodialysis patients were treated with regular haemodialysis 4.5–5 h three times a week. The vitamin D analogue was administered 3–10 h after dialysis and no haemodialysis was performed during the study. In both haemodialysis and peritoneal dialysis the calcium concentration of the dialysis fluid was 1.75 mmol/l. Calcium carbonate and/or aluminum-aminoacetate (87 mg/ml) were administered routinely to all patients to reduce plasma-phosphate levels. In the 1,25(OH)2D3 group, six patients were treated with calcium carbonate and three patients further needed a small dose of aluminum-aminoacetate. In the 1{alpha}(OH)D3 group four patients were treated with calcium carbonate and one patient was supplemented with aluminum-aminoacetate. One patient was treated with aluminum-aminoacetate only. The mean dose of calcium carbonate in the 1,25(OH)2D3 group was 3.41±0.5 g/day and in the 1{alpha}(OH)D3 group 3.75±0.97 g/day (n.s.). The mean dose of aluminum-aminoacetate in the 1,25(OH)2D3 group was 60±15 ml/day and in the 1{alpha}(OH)D3 group 23±8 ml/day (n.s.).

None of the healthy volunteers had been treated previously with 1,25(OH)2D3 or 1{alpha}(OH)D3. One patient in the 1,25(OH)2D3 group received 1{alpha}(OH)D3 at a dose of 1 µg i.v. once a week until 1 week before the study. In the 1{alpha}(OH)D3 group one patient received 1{alpha}(OH)D3 orally at a dose of 1 µg every second day until 3 months before, one patient 0.5 µg orally once daily until 6 weeks before, and one patient 1 µg once a day until 1 week before the study. No active vitamin D analogues were administered between the two study periods. Further, the patients in the 1,25(OH)2D3/1{alpha}(OH)D3 group were treated with: prednisolone 0/1, isosorbidnitrat 0/1, digoxin 0/3, Ca-channel blockers 4/2, pinacidil 1/0, metoprolol 2/0, furosemid 1/5, potassium 0/1, acetylsalicylic acid 0/1, terfenadin 1/0, cyproheptadin 0/1, quinin 1/0, warfarin 1/1, anxiolytika 0/4, insulin 0/1, cephalexin 0/1, netilmicin 0/1, ferro 2/0, erythropoietin 6/0, omeprazol 3/0, cimetidin 0/4, and phenytoin 0/1. All patients gave their informed consent to participate in the study, which was approved by the Danish Ethical Committee for Medical Research.

Treatment schedule
Healthy volunteers
Initially an indwelling i.v. catheter was placed in both cubital veins: one for administration of 1{alpha}(OH)D3 or 1,25(OH)2D3 and the other for blood sampling. Two sets of blood samples were obtained initially as basal values. Then at time zero, 4 µg of 1,25(OH)2D3 (Calcijex®, Abbott) was injected as a bolus. Blood samples for analysis of plasma-Ca2+, plasma-1,25(OH)2D3 and plasma-PTH were drawn at time 0, 0.25, 0.5, 1, 2, 4, 6, 9, 12, 24, 48, and 72 h. With an interval of 2 weeks, the same schedule was performed in the same healthy volunteer, but with administration of 4 µg of 1,25(OH)2D3 orally (Rocaltrol®, Roche), 4 µg of 1{alpha}(OH)D3 intravenously (Etalpha®, Leo, Denmark) and 4 µg of 1{alpha}(OH)D3 orally (Etalpha®), respectively. This means that the single healthy volunteer was studied in all four protocols.

Uraemic patients
Because of the need to obtain 10 ml of blood for each analysis of 1,25(OH)2D3 the uraemic patients had to be divided into two groups—one group that had 4 µg of 1{alpha}(OH)D3 orally and then intravenously or vice versa (the 1{alpha}(OH)D3 group, n=6) and another group that had 4 µg of 1,25(OH)2D3 orally and then intravenously or vice versa (the 1,25(OH)2D3 group, n=6).

Exactly the same schedule as described for the healthy volunteers was then performed with administration of 4 µg of 1{alpha}(OH)D3 intravenously and orally (Etalpha®) for the 1{alpha}(OH)D3 group and 4 µg of 1,25(OH)2D3 intravenously (Calcijex®) and orally (Rocaltrol®) for the 1,25(OH)2D3-group.

The producers of the different vitamin D analogues have reported that the variation between the designated and the actually delivered dose for inj. Calcijex® (Abbott) was 90–125%, 95–115% for caps. Rocaltrol®, 95–105% for inj. Etalpha®, and 90–115% for caps. Etalpha®.

At the beginning of each session, blood samples were analysed for total plasma-calcium, plasma-inorganic phosphate (Pi), plasma-alkaline phosphatases, plasma-alanine aminotransferase (ALAT), plasma-lactate dehydrogenase (LDH), plasma-coagulation factor II, VII, X (PP), and plasma-albumin and after 72 h again also for total plasma-calcium and plasma-Pi.

Methods
The 1,25(OH)2D3 was extracted from plasma with diethyl ether, and extracts were chromatographed. 1,25(OH)2D3 was measured by a competitive protein binding assay using calf thymus cytosol as the source of binding protein. The sensitivity is 3 pg/ml, the intra-assay variation 3.8% and the inter-assay variation 14.7%. In our laboratory the normal range is 37.7±11.2 pg/ml (±SD).

Plasma-PTH 1-84 was measured by an immunoradiometric assay (IRMA, Allegro, from Nichols Institute, USA). The sensitivity is 1 pg/ml, the intra-assay variation is 3%, and the inter-assay variation 6%. The normal range in our laboratory is 10–55 pg/ml, determined in 255 normal subjects.

Plasma-Ca2+ and pH were measured by a calcium ion electrode analyser (ICA 2, Radiometer, Copenhagen, Denmark). Total plasma-calcium and plasma-inorganic Pi were measured by photometry. Plasma-alkaline phosphatase, plasma-ALAT, plasma-LDH, plasma-PP, and plasma-albumin were measured by standard laboratory tests. Blood samples for determination of plasma-Ca2+ and pH were measured immediately. Blood samples for the determination of plasma-1,25(OH)2D3 and plasma-PTH were immediately placed on ice, centrifuged at 4°C, and stored at -20°C until analysis.

Calculations
Calculation of the pharmacokinetics was performed in the following way. After i.v. administration of 1,25(OH)2D3 the concentration vs time curves exhibited a biexponential decline on a semi-logarithmic plot with an initial more rapid distribution phase ({alpha}) and a terminal elimination phase (ß). By extrapolating the exponential curves to time zero, corresponding to the two compartment model, the initial concentrations of 1,25(OH)2D3, corresponding to the phases of C{alpha}0 and Cß0 were determined. The theoretical maximal 1,25(OH)2D3 concentration was then calculated as C0=C{alpha}0+Cß0 and the initial volume of distribution (Vd initial) of 1,25(OH)2D3 as the test dose/C0. The AUC after i.v. as well as oral dosages (AUCi.v. and AUCp.o.) was calculated by trapezoidal integration from zero to 72 h. The bioavailability of 1,25(OH)2D3 after 1,25(OH)2D3 orally was then determined as F=AUCp.o./AUCi.v. and the bioavailability of 1,25(OH)2D3 after oral and i.v. administration of 1{alpha}(OH)D3 as the respective AUCs divided by AUC1,25 i.v.. In uraemia, where the uraemic patients who received 1{alpha}(OH)D3 and 1,25(OH)2D3 were different subjects, the bioavailability of 1,25(OH)2D3 after oral and i.v. administration of 1{alpha}(OH)D3 was calculated as the respective AUCs divided by mean AUC1,25 i.v. obtained in uraemic patients. The clearance (Cl) of 1,25(OH)2D3 was calculated as Cl=test dose/AUC(1,25 i.v.). The final volume (Vd) of distribution of 1,25(OH)2D3 at steady state was calculated as Cl/-k, where k was the elimination rate constant of the ß-phase of the 1,25(OH)2D3 disappearance curve after i.v. administration. The terminal elimination half-life (t1/2) of 1,25(OH)2D3 was calculated as (t1/2)=ln2/-k, where ln2 was the natural logarithm of 2 [11]. Maximal increase in plasma-1,25(OH)2D3 level ({Delta}-maximum plasma-level), time to reach maximal plasma-1,25(OH)2D3 level and time to decrease maximal plasma-1,25(OH)2D3 level by 50% (T50% maximum plasma-level) were calculated as mean±SEM of the results obtained in the single subject.

Statistics
For comparison of mean values during the different study periods, two-way analysis of variance, as described by Altman [12] was used. Guided by these results a paired/unpaired Student's t-test was used for comparison of mean values between specific time periods. A P value <0.05 was considered statistically significant.



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Baseline biochemical parameters (Table 1Go)
The baseline biochemical values for the healthy volunteers were, as expected, all normal. Regarding the uraemic patients, the plasma levels of total calcium, Ca2+, alkaline phosphatases, ALAT, LDH, and PP were within the normal ranges; although within the normal range plasma-alkaline phosphatases were significantly higher in the 1{alpha}(OH)D3 than in the 1,25(OH)2D3 group (P<0.008). Plasma-albumin was lower in the uraemic patients than in the normal subjects (P<0.01), lowest in the 1{alpha}(OH)D3 group. Compared with normal subjects plasma-Pi was elevated (P<0.007) and plasma-1,25(OH)2D3 low (P<0.0001) in both uraemic groups, which were not significantly different from each other. Plasma-PTH was 2.5–3 times elevated in both uraemic groups (P<0.006). No significant difference was observed between the PTH levels of the two uraemic groups or between the patients treated either by CAPD or by haemodialysis within the 1,25(OH)2D3 group (CAPD/haemodialysis 112.9±1.3/295.5±55.7 pg/ml, n.s.) or 1{alpha}(OH)D3 group (CAPD/haemodialysis 102.3±81.7/172.4±7.6, n.s.).


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Table 1.  Clinical data and baseline biochemical parameters prior to i.v. and oral administration of 1,25(OH)2D3 and 1{alpha}(OH)D3 to healthy volunteers and uraemic patients

 

Pharmacokinetics of 1,25(OH)2D3 after 1,25(OH)2D3 or 1{alpha}(OH)D3 administration to healthy volunteers
The plasma-1,25(OH)2D3 concentrations vs time after i.v. and oral administration of 4 µg of 1,25(OH)2D3 and 4 µg of 1{alpha}(OH)D3 are summarized in Figure 1Go. A semi-logarithmic transformation of the plasma-1,25(OH)2D3 i.v. course is inserted in Figure 1Go.



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Fig. 1.  Plasma-1,25(OH)2D3 concentrations (mean±SEM) in relation to time in six healthy volunteers following i.v. and oral administration of 4 µg of 1,25(OH)2D3 and i.v. and oral administration of 4 µg of 1{alpha}(OH)D3. The same six subjects were included in all four parts of the study. A semi-logarithmic transformation of the plasma-1,25(OH)2D3 course after i.v. administration of 1,25(OH)2D3 is inserted at the right corner.

 
1,25(OH)2D3 given i.v. resulted in a theoretical maximal increase of the plasma-1,25(OH)2D3 concentration (C0) of 412.9±25.3 pg/ml (Table 2Go). The increase in plasma-1,25(OH)2D3 measured after 15 min was 366.5±18.8 pg/ml. A decline in plasma-1,25(OH)2D3 to levels that were not significantly different from baseline levels, was achieved after 24 h (Figure 1Go). The terminal elimination half-life (t1/2) was 25.9±3.5 h (Table 2Go). The time to reduce maximal plasma-1,25(OH)2D3 concentrations by 50% was 5.1±1.3 h (Table 2Go).


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Table 2.  Pharmacokinetic parameters for 1,25(OH)2D3 in six healthy volunteers and 12 uraemic patients after a single bolus administration of either 4 mg 1,25(OH)2D3 or 4 mg 1{alpha}(OH)D3 intravenously (i.v.) or orally (p.o.)

 
The distribution was fitted to a two-compartment model. Within the first hour the initial volume of distribution (Vd initial) constituted 0.15±0.01 l/kg body weight and the final volume of distribution (Vd) 0.49±0.14 l/kg body weight (Table 3Go). The clearance was 23.5±4.3 ml/min (Table 3Go).


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Table 3.  Pharmacokinetic parameters of 1,25(OH)2D3 in six healthy volunteers and six uraemic patients after a single bolus administration of 4 µg 1,25(OH)2D3 intravenously

 
1,25(OH)2D3 given orally resulted in a maximal increase of the plasma-1,25(OH)2D3 concentrations of 223.2±31.3 pg/ml (Table 2Go) after 2.3±0.4 h. A decline in plasma-1,25(OH)2D3 to levels that were not significantly different from baseline levels, was achieved after 24 h (Figure 1Go). The bioavailability was 70.6±5.8% (Table 4Go). t1/2 was 28.2±3.5 h—not significantly different from the result obtained after i.v. administration. The time to reduce the maximal plasma-1,25(OH)2D3 concentrations by 50% was 10.5±1.2 h, which was significantly longer than that of 1,25(OH)2D3 given i.v. (P<0.03).


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Table 4.  Bioavailability of 1,25(OH)2D3 in six healthy volunteers after a single bolus of 4 mg of 1,25(OH)2D3 or 4 µg of 1{alpha}(OH)D3 intravenously (i.v.) and orally (p.o.)

 
1{alpha}(OH)D3, given i.v., resulted in a maximal increase of the plasma-1,25(OH)2D3 concentrations of 63.0±15.8 pg/ml (Table 2Go) reached after 7.8±1.5 h. A decline in plasma-1,25(OH)2D3 to levels not significantly different from the baseline levels was achieved after 24 h (Figure 1Go). Bioavailability of 1,25(OH)2D3 after i.v. 1{alpha}(OH)D3 administration was 42.4±10.9% (Table 4Go). t1/2 of 1,25(OH)2D3 was 48.6±4.0 h (Table 2Go). The time to reduce the maximal plasma-1,25(OH)2D3 concentration by 50% was 34.8±7.2 h (Table 2Go).

1{alpha}(OH)D3 given orally resulted in a maximal increase of the plasma-1,25(OH)2D3 concentrations of 70.9±23.4 pg/ml (Table 2Go) reached after 6.5±1.5 h. A decline in plasma-1,25(OH)2D3 to levels not significantly different from baseline levels was achieved after 24 h (Figure 1Go). The bioavailability of 1,25(OH)2D3 after oral administration of 1{alpha}(OH)D3 was 43.8±9.2% (Table 4Go). t1/2 of 1,25(OH)2D3 was 47.1±4.0 h. The time to reduce maximal plasma-1,25(OH)2D3 concentration by 50% was 37.7±9.0 h (Table 2Go). The results obtained after oral administration of 1{alpha}(OH)D3 were very similar to and not significantly different from the results obtained after i.v. 1{alpha}(OH)D3 administration.

Pharmacokinetics of 1,25(OH)2D3 after 1,25(OH)2D3 or 1{alpha}(OH)D3 administration to chronically uraemic patients
The plasma-1,25(OH)2D3 concentrations vs time after i.v. and oral administration of 4 µg of 1,25(OH)2D3 and 4 µg of 1{alpha}(OH)D3 are summarized in Figure 2Go. A semi-logarithmic transformation of the plasma-1,25(OH)2D3 i.v. course is inserted in Figure 2Go.



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Fig. 2.  Plasma-1,25(OH)2D3 concentrations (mean±SEM) in relation to time in six chronically uraemic patients following i.v. and oral administration of 4 µg of 1,25(OH)2D3 and in another six chronically uraemic patients following i.v. and oral administration of 4 µg of 1{alpha}(OH)D3. A semi-logarithmic transformation of the plasma-1,25(OH)2D3 course after i.v. administration of 1,25(OH)2D3 is inserted at the right corner.

 
1,25(OH)2D3 given i.v. to uraemic patients with a body weight similar to that of healthy volunteers resulted in a theoretical maximal increase of the plasma-1,25(OH)2D3 concentrations (C0) of 360.6±38.4 pg/ml—a level not significantly different from that obtained in healthy volunteers (Table 2Go). The increase in plasma-1,25(OH)2D3 measured after 15 min was 337.0±39.5 pg/ml. The following decline in plasma-1,25(OH)2D3 was slower than that of healthy volunteers with significantly higher plasma-1,25(OH)2D3 levels after 12, 24, 48, and 72 h (P<0.006, P<0.01, P<0.0005, and P<0.05) in the uraemic patients (Figure 3Go). Plasma-1,25(OH)2D3 declined to near baseline levels after 72 h, but were still significantly higher (P<0.04) than baseline levels (Figure 2Go). t1/2 was 21.6±3.5 h, and the time to reduce the maximal plasma-1,25(OH)2D3 concentration by 50% was 8.7±1.3 h (Table 2Go)—not significantly different from healthy volunteers. The slower initial decline of plasma-1,25(OH)2D3 resulted in a clearance of 1,25(OH)2D3 of 10.1±1.4 ml/min—57% lower than in healthy volunteers (P<0.03) (Table 3Go). The initial and final volumes of distribution were similar to those observed in healthy volunteers (Table 3Go).



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Fig. 3.  Comparison between the increase in plasma-1,25(OH)2D3 concentrations (measured-baseline value, mean±SEM) in six healthy volunteers and 12 uraemic patients in relation to time following i.v. and oral administration of 4 µg of 1,25(OH)2D3 and i.v. and oral administration of 4 µg of 1{alpha}(OH)D3. The six healthy volunteers were included in all four parts of the study. The uraemic patients were separated into two groups, the 1{alpha}(OH)D3 group and the 1,25(OH)2D3-group, due to the great amount of blood needed for analysis of plasma-1,25(OH)2D3. Significant differences (P<0.05) between normal controls and uraemic patients are marked by *.

 
1,25(OH)2D3 given orally resulted in a maximal increase of plasma-1,25(OH)2D3 concentrations of 227.9±50.4 pg/ml, reached after 3.15±1.2 h—equal to the levels obtained in healthy volunteers (Table 2Go). Plasma-1,25(OH)2D3 declined to levels not significantly different from baseline levels after 72 h (Figure 2Go). In accordance with the lower clearance in uraemic patients the AUC was 228% higher than that of healthy volunteers (P<0.003). However, the bioavailability in uraemic patients was 72.2±4.8%, not significantly different when compared with healthy volunteers (Table 4Go). t1/2 was 26.9±5.6 h, not significantly different from healthy volunteers. The time to reduce maximal plasma-1,25(OH)2D3 concentration by 50% was 15.3±1.5 h, significantly longer than in healthy volunteers (P<0.03).

1{alpha}(OH)D3 given i.v. resulted in a maximal increase of plasma-1,25(OH)2D3 of 147.0±17 pg/ml after 3.5±0.6 h, significantly higher than observed in healthy volunteers (P<0.005) (Table 2,Go Figure 3Go). Plasma-1,25(OH)2D3 declined to levels not significantly different from baseline levels after 72 h (Figure 2Go). The bioavailability of 1,25(OH)2D3 was 42.0±2.0% (Table 4Go), not different from healthy volunteers, but significantly lower than following administration of 1,25(OH)2D3 (P<0.03). t1/2 of 1,25(OH)2D3 was 36.7±6.7 h (Table 2Go) not significantly different from healthy volunteers. The time to reduce the maximal plasma-1,25(OH)2D3 concentration by 50% was 14.3±3.5 h, which was significantly faster than after oral administration (P<0.01) and significantly faster than in healthy volunteers (P<0.03) (Table 2Go).

1{alpha}(OH)D3 given orally resulted in a maximal increase of plasma-1,25(OH)2D3 concentrations of 73.0±12.5 pg/ml after 7.6±1.4 h (Table 2Go) a concentration vs time curve similar to that of healthy volunteers (Table 3Go). The peak levels were significantly lower than after i.v. administration (P<0.005) (Tables 2Go and 3Go). Plasma-1,25(OH)2D3 declined to levels not significantly different from baseline levels after 48 h (Figure 2Go). The bioavailability was 29.7±3.1% (Table 4Go), not significantly different from that of healthy volunteers. t1/2 of 1,25(OH)2D3 was 29.2±5.2 h (Table 2Go), and the time to reduce maximal plasma-1,25(OH)2D3 by 50% was 32.1±5.6 h—not significantly different from that of healthy volunteers.

PTH
The baseline PTH levels in the healthy volunteers were within the normal range (23.5±0.33 pg/ml, Table 1Go), and moderately elevated in the uraemic patients (P<0.006) (1,25(OH)2D3 group 137.3±33 pg/ml, 1{alpha}(OH)D3 group 173.3±84 pg/ml, Table 1Go), with no significant difference between the two uraemic groups.

The baseline PTH levels, which were obtained at the beginning of each of the four different protocols in the healthy volunteers and in each of the two different protocols in the 1{alpha}(OH)D3 and 1,25(OH)2D3 group, were not significantly different from each other. The baseline PTH values are therefore presented as the mean PTH values obtained at the beginning of each study (Table 1Go). The changes in PTH after administration of 4 µg of each of the two vitamin D metabolites are presented as per cent of baseline PTH levels (Figure 4Go). In the healthy volunteers, PTH levels were significantly lower than baseline levels 24 h after administration of 4 µg of 1,25(OH)2D3 i.v. (P<0.004), after oral administration of 1,25(OH)2D3 at 9, 12, and 24 h (P<0.05, P<0.03, and P<0.03) and after oral administration of 1{alpha}(OH)D3 at 24 h (P<0.0005) (Figure 4Go). 1{alpha}(OH)D3 i.v. had no suppressive effect on plasma-PTH levels within 24 h.



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Fig. 4.  Mean values (mean±SEM) of plasma-Ca2+ and plasma-intact PTH in relation to time following i.v. and oral administration of 4 µg of 1,25(OH)2D3 and i.v. and oral administration of 4 µg of 1{alpha}(OH)D3. The same six healthy volunteers were included in all four parts of the study. The uraemic patients were separated into two groups, the 1{alpha}(OH)D3 group and the 1,25(OH)2D3 group due to the great amount of blood needed for analysis of plasma-1,25(OH)2D3. A paired t-test was performed guided by two-way analysis of variance. Values significantly different form baseline value (P<0.05) are marked below the curve by the symbol of the relevant vitamin D analogue. Plasma-PTH values are presented as percentage of baseline value due to great variation in basal plasma-PTH levels.

 
In the uraemic patients the PTH levels were significantly lower than basal levels after administration of 1,25(OH)2D3 i.v. at 24, 48, and 72 h (P<0.04, P<0.008, and P<0.01), after administration of 1,25(OH)2D3 orally at 24, 48, and 72 h (P<0.002, P<0.0003, and P<0.002) and after administration of 1{alpha}(OH)D3 i.v. at 24, 48, and 72 h (P<0.02, P<0.002, and P<0.02, Figure 4Go).

Using two-way analysis of variance, no significant changes in plasma-PTH levels were observed in healthy volunteers after administration of 4 µg of 1{alpha}(OH)D3 i.v. and in uraemic patients after administration of 4 µg of 1{alpha}(OH)D3 orally within the 72 h. When the suppression of plasma-PTH was compared after administration of 1,25(OH)2D3 and 1{alpha}(OH)D3 orally in healthy volunteers and after administration of 1,25(OH)2D3 and 1{alpha}(OH)D3 i.v. in uraemic patients, no significant differences were observed (Figure 4Go).

Plasma-Ca2+
Plasma-Ca2+ was initially within normal limits in both healthy volunteers and uraemic patients (Table 1Go). No significant changes were seen in the healthy volunteers while a small, but significant increase of plasma-Ca2+ was observed in the uraemic group 48 and 72 h after administration of 1,25(OH)2D3 i.v. (P<0.02 and P<0.02) and 12, 24, and 48 h after administration of 1,25(OH)2D3 orally (P<0.02, P<0.007, and P<0.02) (Figure 4Go).



   Discussion
 Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
The present study compared the pharmacokinetics of 1,25(OH)2D3 and 1{alpha}(OH)D3 in response to i.v. and oral administration of a single dose in both healthy volunteers and uraemic patients. In order to set the present results into perspective, we have summarized the relatively sparse and widely scattered results on human subjects, which previously have been reported in the literature in Tables 5GoGo7Go.


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Table 5. Pharmacokinetic parameters of 1,25(OH)2D3 intravenously vs 1,25(OH)2D3 orally obtained in the present study and compared to results reported in the literature after single dose administration

 

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Table 6.  Pharmacokinetic parameters of 1{alpha}(OH)D3 intravenously vs 1{alpha}(OH)D3 orally obtained in the present study and compared to results reported in the literature after single dose administration

 

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Table 7.  Pharmacokinetic parameters of 1,25(OH)2D3 vs 1{alpha}(OH)D3 obtained in the present study and compared with results reported in the literature after single dose administration

 
In the present study a significantly lower clearance (Cl) of 1,25(OH)2D3 was found in uraemic patients than in healthy volunteers (10.1±1.4 vs 23.5±4.3 ml/min, P<0.03). This is in agreement with another study [13] which compared the disappearance of 3H-1,25(OH)2D3 in healthy volunteers and uraemic patients and found a higher clearance of 1,25(OH)2D3 in the healthy volunteers than in uraemic patients (0.6 vs 0.5 ml/min/kg, P<0.001). In a third study a clearance of 1,25(OH)2D3 of 15.3 ml/h/kg was found in terminal uraemic patients [14], similar to that of the present study. This difference in the clearance of 1,25(OH)2D3 between healthy volunteers and uraemic patients might be due to the lack of kidney function, which affects the renal metabolism and elimination of 1,25(OH)2D3, or due to the presence of toxins in plasma of uraemic patients, which suppresses the degradation of 1,25(OH)2D3 [13].

The clearance of 1,25(OH)2D3 was calculated as ‘test dose/AUC(1,25 i.v.)’. The AUC1,25 of both oral and i.v. administration was significantly higher in uraemic patients than in healthy volunteers (P<0.004). This is in accordance with the results of the literature, as stated in Tables 5Go and 7Go. In a single study on uraemic patients, no difference in AUC between oral and i.v. administration of 2 µg of 1,25(OH)2D3 was found [15]. The Vd initial (initial volume of distribution) and final Vd (volume of distribution) of 1,25(OH)2D3 were the same in healthy volunteers and uraemic patients. Final Vd was higher than the Vd initial demonstrating the well known affinity of vitamin D binding globulin for 1,25(OH)2D3.

The maximal plasma-1,25(OH)2D3 levels after oral administration of 1,25(OH)2D3 were significantly lower (P<0.02) than after i.v. administration of 1,25(OH)2D3 to both healthy volunteers and uraemic patients, similar to the calculated AUC levels, reflecting an oral bioavailability of about 70%. This could be explained by incomplete absorption, local intestinal degradation, and/or hepatic metabolism of enterally absorbed 1,25(OH)2D3 (first pass effect).

A comparison of 1{alpha}(OH)D3 given orally and intravenously to the same healthy volunteer has to our knowledge not been reported previously, and only reported once in uraemic patients [16]. The plasma levels of 1,25(OH)2D3 obtained after either i.v. or oral administration of 1{alpha}(OH)D3 were significantly different in the uraemic patients (P<0.03) with the highest plasma peak level obtained after i.v. administration. In the healthy volunteers the plasma levels of 1,25(OH)2D3 after i.v. and oral administration of 1{alpha}(OH)D3 were similar and not different from the results obtained after oral administration of 1{alpha}(OH)D3 to uraemic patients. The reason for this unexpected dissimilarity between the metabolism of 1{alpha}(OH)D3 after i.v. administration to uraemic patients and healthy volunteers is unclear. It is, at the present time, not possible to measure plasma-concentration of 1{alpha}(OH)D3 without administration of radioactive labelled 1{alpha}(OH)D3. In this study the commercially available drug was used and, therefore, it was not possible to calculate the terminal elimination half-life (t1/2) for 1{alpha}(OH)D3. The time involved before the maximal plasma levels of 1,25(OH)2D3 were reduced by 50% (T50% maximum plasma level) was nearly two times longer after oral and i.v. administration of 1{alpha}(OH)D3 than after similar administration of 1,25(OH)2D3 to healthy volunteers (P<0.02). The continuous conversion of 1{alpha}(OH)D3 to 1,25(OH)2D3 is presumably a contributory factor to the longer T50% maximum plasma level after administration of 1{alpha}(OH)D3.

The bioavailability of 1,25(OH)2D3 after administration of 1{alpha}(OH)D3 was only 40% in both normal and uraemic subjects (Table 4Go). The peak levels achieved after oral and i.v. administration of 1{alpha}(OH)D3 were in the present study as described in other studies [46] only about 50% of the plasma levels achieved after similar doses of 1,25(OH)2D3 (Table 7Go). The exact reason for this is not known, but might be due to metabolism of 1{alpha}(OH)D3 to substances other than 1,25(OH)2D3 [17], due to excretion of non-metabolized 1{alpha}(OH)D3, or due to deposition of 1{alpha}(OH)D3. Against these possibilities are, however, that the fecal radioactivity of 1{alpha}(OH)D3 after i.v. administration of 3H-1{alpha}(OH)D3 has been found to be only 5% [17], and the rapid rate of reversal of plasma-calcium and calcium excretion after stop of treatment [18].

The terminal elimination half-life (t1/2) of 1,25(OH)2D3 after administration of 1{alpha}(OH)D3 was in the present, as in other studies [4,16], based upon the elimination rate constant of the ß-phase of the 1,25(OH)2D3 disappearance curve. In the present study on healthy volunteers, t1/2 of 1,25(OH)2D3 was significantly longer after oral and i.v. administration of 1{alpha}(OH)D3 than after oral and i.v. administration of 1,25(OH)2D3 (P<0.02).

In the uraemic patients, t1/2 of 1,25(OH)2D3 was not significantly different between 1{alpha}(OH)D3 and 1,25(OH)2D3, independent of the form of administration in contrast to what has been found in another investigation [4]. Neither in the healthy volunteers nor in the uraemic patients of the present study were significant differences in t1/2 of 1,25(OH)2D3 found between oral and i.v. administration of 1{alpha}(OH)D3.

Only a few studies have measured the plasma-levels of 1,25(OH)2D3, which were generated after administration to the same subject of either 1,25(OH)2D3 or 1{alpha}(OH)D3 (Table 7Go). In the present study, the same dose of 1,25(OH)2D3 and 1{alpha}(OH)D3 was administered to the same healthy volunteer, resulting in significantly higher peak levels of 1,25(OH)2D3 (P<0.009) and significantly greater AUC (P<0.02) after administration of 1,25(OH)2D3 than after similar administration of 1{alpha}(OH)D3. The time to reach peak levels of 1,25(OH)2D3 in healthy volunteers and uraemic patients was significantly shorter after oral administration of 1,25(OH)2D3 than after oral administration of 1{alpha}(OH)D3 (healthy volunteers: 2.3±0.4 vs 6.5±1.5, P<0.05; uraemic patients: 3.5±1.2 vs 7.6±1.4 h, P<0.05), presumably reflecting the time used to convert 1{alpha}(OH)D3 to 1,25(OH)2D3. The results obtained in the present study are, however, generally in accordance with the scattered previously reported results, as indicated in Table 7Go.

The acute effect of a single dose of 1,25(OH)2D3 compared with 1{alpha}(OH)D3 on the suppression of PTH secretion in healthy volunteers has only sporadically been studied—and to our best knowledge not in a direct comparative set-up as in the present study. In healthy volunteers the results of previous investigations did not demonstrate any acute suppressive effect on PTH secretion for up to 12 h of 1,25(OH)2D3 i.v. [19], 1,25(OH)2D3 orally [20], or 1{alpha}(OH)D3 i.v. [21] at doses from 1.5 to 2.7 µg.

In chronically uraemic children with persistent HPT a suppression of PTH levels 12 h after administration of a single dose of 1.5 µg/m2 of 1,25(OH)2D3 orally and i.v. [22] has been demonstrated previously. The same was found in patients on CAPD with persistent HPT after a single dose of 80 ng/kg of 1{alpha}(OH)D3 orally or i.v. [23]. In patients on chronic haemodialysis with persistent HPT, several studies have demonstrated a direct suppressive effect on PTH secretion after intermittent administration two to three times a week of 1,25(OH)2D3 or 1{alpha}(OH)D3 [1,2,9]. One study demonstrated a suppression of PTH secretion 48 h after administration of a single dose of 8 µg of 1,25(OH)2D3 i.v. [24], while another study did not observe any effect for 44 h after administration of a single dose of 4 µg of 1{alpha}(OH)D3 [8]. Neither 4 µg of 1,25(OH)2D3 or of 1{alpha}(OH)D3 given i.v. as a single shot immediately before dialysis caused any suppression of plasma-PTH during dialysis [25]. The present study found in healthy volunteers a transitory suppressive effect on plasma-PTH levels 12–24 h after 4 µg of oral 1{alpha}(OH)D3 and 12–24 h after oral as well as i.v. 1,25(OH)2D3. In uraemic patients a similar significant suppression of plasma-PTH levels of i.v. 1{alpha}(OH)D3 and of i.v. as well as oral 1,25(OH)2D3 was observed, which lasted for at least 72 h. This suppressive effect in uraemic patients of i.v. 1{alpha}(OH)D3 and 1,25(OH)2D3 on the plasma-PTH levels was not significantly different. This is in accordance with the clinical experience that 1{alpha}(OH)D3 and 1,25(OH)2D3 are both effective drugs in the treatment of sec. HPT [13]. Presumably, due to the small number of patients included, no suppressive effect on plasma-PTH levels was observed after administration of 1{alpha}(OH)D3 i.v. to healthy volunteers and of 1{alpha}(OH)D3 orally to uraemic patients. The concomitantly small, but significant increase in plasma-Ca2+ observed in the uraemic patients after administration of 1,25(OH)2D3 does however not allow for any specific conclusions to be made on the exact therapeutic equivalence of the two drugs.

As the peak concentration of 1,25(OH)2D3 after administration of 1{alpha}(OH)D3 was markedly lower than that obtained after similar doses of 1,25(OH)2D3, either the peak concentration of 1,25(OH)2D3 is probably of less importance for the direct suppression of PTH secretion than assumed previously, or the effect of 1{alpha}(OH)D3 on the PTH secretion in uraemic patients cannot alone be explained by the conversion of 1{alpha}(OH)D3 to 1,25(OH)2D3. Therefore, the theoretical possibility exists that the 25-hydroxyl group might not be mandatory for the activity of vitamin D3 in the parathyroid glands, but that the 1{alpha}-hydroxyl group is the structural feature required for the expression of the hormonal activity on PTH gene suppression. This assumption is supported by results from an in vitro study from our laboratory where the suppression of PTH secretion from bovine parathyroid cells by 1{alpha}(OH)D3 was equal to that of 1,25(OH)2D3 [26].

In conclusion, the present study demonstrated a 57% lower metabolic clearance rate of 1,25(OH)2D3 in uraemic patients, as compared with healthy volunteers (P<0.03). The bioavailability of 1,25(OH)2D3 following administration of 1{alpha}(OH)D3 i.v. and orally to both healthy volunteers and uraemic patients was markedly lower than after administration of oral 1,25(OH)2D3 (P<0.03). In spite of lower plasma-1,25(OH)2D3 levels after administration of 1{alpha}(OH)D3, no significant difference was observed on the suppressive effect of 4 µg intravenously of either 1,25(OH)2D3 or 1{alpha}(OH)D3 on the plasma-PTH levels in the uraemic patients. This might suggest the existence of an effect of 1{alpha}(OH)D3 on the parathyroid glands which is independent of the plasma-1,25(OH)2D3 levels, that are achieved after oral or i.v. administration of 1{alpha}(OH)D3.



   Acknowledgments
 
We thank Ms Betty Fischer and Ms Karen Meibom for their invaluable laboratory assistance.



   Notes
 
Correspondence and offprint requests to: Lisbet Brandi, MD, Med. Department P 2131, Rigshospitalet, 9 Blegdamsvej, DK-2100 Copenhagen, Denmark. Email: Lisbet.Brandi{at}dadlnet.dk Back



   References
 Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 

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Received for publication: 27.10.00
Accepted in revised form: 17.11.01