Pharmacokinetics of Vitamin B6 Supplements in ...

Review Article

Pharmacokinetics of Vitamin B6 Supplements in Humans Janos Zempleni, PhD Department of Biochemistry, School of Medicine, Emory University, Atlanta, Georgia

Key words: human, pharmacokinetics, pyridoxal, pyridoxamine, pyridoxic acid, pyridoxine The use of vitamin B6 supplements is widespread today. Doses used are often elevated far above the physiological range and reach level s up to 600-fold higher than recommended dietary allowances for healthy people. While the toxic effects caused by chronic high doses of vitamin B6 have been described earlier, pharmacokinetic data on vitamin B6 supplements are rare. This article reviews the pharmacokinetic data of vitamin B6 from human subjects.

Key teaching points: • • • • • •

There is no indication of circadi an rhythm in vitamin B6-levels in blood plasma. Pharmacokinetics of vitamin B6 differ fundamentally, depending from the route of administration. The rate constants of elimination of B6-metabolites indicate a tendency of accumulation. Pyridoxine is rapidly metabolized in erythrocytes. 4-Pyri doxic acid and pyridoxine are secreted in renal tubulu s, whereas pyridoxal is reabsorbed. Metabolism of vitamin B6 is altered in premature infants.

containing supplements, either as multivitamin preparation or as single vitamin [16]; doses up to 1000 mg/day vitamin B6 were used [17]. Pharmacokinetic data are imperative to optimize these sup­ plements concerning their dose, route of administration, and formulation factors. Availability of such data is even more desirable, as toxic effects (ataxia, severe sensory-nervous-sys­ tem dysfunction) have been described for chronic high-dosed consumption of vitamin B6 [18]. Such side-effects were ob­ served when consuming doses of 150-200 mg/day for several weeks [19,20]. Recently capable laboratory procedures have been devel­ oped which allow detection of all the physiological relevant metabolites in body fluids [21-24]. They induced some inves­ tigations on the pharmacokinetics of vitamin B6 in humans. The available data are summarized in the following sections. Stud­ ies on animals are provided in cases, when the corresponding data in humans are lacking. The metabolic interconversions of vitamin B6 in humans are summarized in Fig. 1.

INTRODUCTION Vitamin B6 supplements are of multipurpose use today. Moderate oral doses are given in order to meet enhanced requirements as apparent in pregnancy or lactation [1-3]. Also, doses in the range of 1 g/day and above are prescribed [4]. This latter megavitamintherapy uses pharmacological properties of vitamin B6 for treatment of enzyme defects such as cystathi­ oninuria, homocysteinemia, and primary oxalosis (type I) [4- 6] . Doses used for these purposes are in some cases ele­ vated more than 600-fold over recommended dietary allow­ ances for healthy adults [7]. In parenteral nutrition these prep­ arations are well-established, too. Doses in a magnitude of up to 100 mg/day are common [8]. The use of vitamin B6 supple­ ments has been of benefit in cases of alcoholism [9 - 11] and patients on chronic hemodialysis [1 2-1 4]. The frequency of vitamin and mineral supplement use has been reported to be 35% to 40% in the United States [15,16]. Thirty percent of the adult population consumed vitamin B6

Abbreviations: AUe = area under the vitamin concentration in plasma versus time curve, C max = maximal concentration, EAST = erythrocyte aspartate aminotransferase, ka = apparent first-order absorption rate constant, K = apparent first-order elimination rate constant (one-compartment model), ko = first-orde r fast disposition rate constant (two-compartment model), k~ = first-order slow disposition rate con stant (two-compartment model). 4-PA = 4-pyridoxic acid, PM(P) = pyridoxamine (5' -phosphate), PN(P) = pyridoxine (5 ' -phosphate), PN . HCI = pyridoxine hydrochloride, PL(P) = pyridoxal (5' -phosphate). tmax = time of maximal concentration.

Address reprint requests to: Janos Zempleni, PhD , Arkansas Children' s Hospital, 800 Marshall Street (Mail slot #512), Little Rock, AR 72202-3591.

Journal of the American College of Nutrition, Vol. 14, No.6, 579-586 (1995) Published by the American College of Nutrition 579

Pharmacokinetics of Vitamin B6 4-Pyridoxic acid

r

AD AO

p:~'fFe

0

'P::ifr::' ==O =T====' pY:~YFmine ' ='

PY ri doxine 5'-phosp hate -SL...., Pyridoxal 5'-phosphate~Pyrid oxamine 5'-phosphate T

Fig. 1. The metabol ic interconversions of vitamin B6 and its main urinary metaboli te 4- pyridoxic acid in humans (abbreviations used: AD = aldehyde dehydrogenase (EC 1.2.1.3); AO = aldehyde oxi ­ dase (EC 1.2.3.1 ); 0 = pyridoxamine 5' -phosphate oxidase (EC 1.4.3.5); P = alkaline phosphatase (EC 3.1.3.2); PK = pytidoxal kinase (EC 2.7.1.35); T = transaminases (EC 2.6.l.x)) [25-30].

CIRCADIAN VARIATIONS IN B6 -METABOLITE CONCENTRATIONS Circadian variations in metabolite concentrations in body fluids and tissues must not be underestimated in pharmacoki­ netics. As opposed to drugs which cannot be detected in body fluids before their administration, vitamins are so-called sys­ temic compounds which means they are detectable in body fluids (baseline values) caused by regular dietary uptake. As pharmacokinetic studies quantitate test doses administered to organisms [31], these baseline values must not be included in calculation of pharmacokinetic parameters. They are not a part of the systemic load of organism by test dose. Normally this is taken into account by su btracting the initially determined base­ line values from the concentrations as observed after adminis­ tration of the tes t dose. This procedure is correct as long as the baseline values keep constant throughout the day , i.e. , no circadian rhythm is detectable. Sometimes the variations in baseline concentrations are negligible compared to the changes in concentrations which are challenged by application of the test dose. If it is not the case, large errors are introduced in calculation of kinetic parameters. It is not sufficient then to subtract a constant baseline value and the circadian variations must be taken into account for analysis. There are only a few studies available on vitamin B6 in this respect. They are dealing with the circadian variation of pyridoxal 5' -phosphate (PLP) in blood serum [32], the constancy of aldehyde forms PLP and pyridoxal (PL) in bl ood plasma [33], and with the variation of urinary excretion of 4-pyridoxic acid (4-PA) l34]. Leinert et al determined the concentration profile of PLP in blood serum in 8 male students over a period of 24 hours (6 samples/person) [32]. Alimentary uptake of vitamin B6 was 2 mg within these 24 hours, distributed upon 4 meals. Large interindividual vari­ ations were observed in PLP concentrations (6 - 16 ng/ml), but intraindividual PLP levels were constant within the observation period. This is in agreement with the results obtained by Mas­ cher [33]. He has measured the concentrations of aldehyde

forms of vitamin B6 in blood plasma in 14 female and 2 male volunteers (20 to 40 years of age). In this investigation, PLP and PL were assessed as total PL after hydrolysis of PLP using acid phosphatase. No variation in concentration was observed over a period of 24 hours. Investigations of urinary excretion of 4-PA in 16 normal­ weight males (22 to 28 years old) demonstrated a weakl y pronounced day-profile of this metabolite [34] . Excretion was significantly lower in the night (minimum 03 .00-04.00 a.m.; 0.82 mg 4-PAIg creatinine) as compared to the time after lunch (maximum 01.00 - 02.00 p.m.; 1.34 mg 4-PAIg creatinine). These variations were attributed to alimentary vitamin con­ sumption rather than a hormonal control by the investigator. Further studies dealing with the circadian rhythm of metab­ olites other than those mentioned above are lacking for blood plasma and urine. The same is true for vitamin B6 in tissues. Therefore no final judgement of circadian variations in vitamin B6 levels is yet possible. In spite of this, it is permissible to assume that diurnal variations in vitamin B6 concentrations in body fluids can become neglected for pharmacokinetic inves­ tigations. This is, as concentrations of B6-metabolites are ele­ vated several fold after a test-dose, irrespective of the route of administration (see below). According to the present knowl­ edge, it is sufficient to subtract a constant pre-dose concentra­ tion.

PHARMACOKINETICS IN BLOOD PLASMA AFTER ORAL OR INTRA VENOUS VITAMIN B6 -ADMINISTRATION In pharmaceutical preparations, vitamin B6 is generally used as pyridoxine hydrochloride (PN ' He l) [8] . Speitling has in­ vestigated the metabolism of 600 mg PN . HCI given orally to 9 healthy young males (24 to 31 years of age) [35]. With in 0.3 hours after administration (i.e., the first blood sample drawn) pyridoxine (PN) had entered the systemic circulation. Speitling used Bateman's function 1 to describe the entrance of PN into central circulation. An apparent first-order absorption rate con­ stant of 2.101 :!:: 0.513 hour- t was obtai ned thereby. This equals a half-life of absorption of 0.354 :!:: 0 .114 hours. Ap­ plying Bateman ' s function is proved by the non-limited absorp­ tion of PN in doses of 600 mg [37]. Therefore procedures of linear pharmacokinetics were applied. Speitling calculated the proportion of PN, which left the liver in non-metabol ized fo rm, to amount to 72- 80% of the dose administered [35] . Retention or metabolism of PN in liver therefore is not a dominant role, even if the enzymatic capacity for metabolism is sufficient [29,38]. PN which leaves the liver is rapidly spread over its

1 Formula llsed for the calculation of th e pharmacokinetic parameters are given in th e Appendix. Bateman'5 function was first used to describe the build-up of a radioactive compound out of a radi oactive parent substance; the new compound exhibited a mono-exponential decay [36].

580

VOL. 14, NO. 6

Pharmacokinetics of Vitamin B6 apparent volume of distribution . This volume (0.60 ± 0.26 Llkg body weight) seems to equal total body water as was shown for young males [39]. After oral administration of PN . HCl, its metabolites PN, PLP, PL, and 4-PA were observed to occur in blood plasma in high concentrations [35]. Detection of pyridoxine 5' -phosphate (PNP) was not possible by the HPLC-method used, whereas pyridoxamine (PM) and pyridoxamine 5' -phosphate (PMP) failed to reach detectable concentrations . Values of maximal concentration (C max ), time of maximal concentration (t max )' and rate constant of disposition are summarized in Table 1. The apparent first-order elimination rate constant (K) was computed from the descending slope of the concentration-time curves using non-linear regression analysis. It can easil y be trans­ formed into half-life tll2 according to: t l/2 = ln2/K [36]. The slow elimination of PLP in its {3-Phase explains its accumula­ tion in the human organism and its potential chronic toxicity [18 - 20]. Protein-binding plays a dominant role in distribution and elimination of the B6 -metabolites. PLP is nearly completely protein-bound in plasma [40,41] , while PL is only partly bound and PN exists completely free [40]. In opposite to other metabolites, the concentration of PLP in plasma cannot be enhanced to any degree by increasing the dose of PN ingested . This was shown by Edwards et al for healthy volunteers (n = 7 to n = 10 per group), who received PN . HCI-doses of up to 800 mg per day for I week [42]. Four hours after the last dose, the concentration of PLP in blood plasma was assessed. Doses of 10 - 800 mg PN · HCI resulted in PLP concentrations in the range of 518 ± 130 to 732 ± 202 nmollL, which were not related to the dose administered (800 mg PN . HCI resulted in a level of 644 ± 182 nmollL PLP). In opposition to this the concentrations ofPL, PN, PNP, and 4-PA in plasma rose steadily in relationship to the PN-dose admin­ istered. Control group (without PN . HC1) showed concentra­ tions of 73 ± 34 nmollL PLP. Similar results were obtained for

9 healthy females, who received doses of 10-100 mg PN· HCl [431. Higher levels ofPLP (up to 3000 nmollL) were observed in plasma of patients suffering from hypophosphatasia [41l. This indicates the predominant role of alkaline phosphatase in regulating the concentration of PLP. The bioavailability of vitamin B6 depends upon which me­ tabolite is present. Wozenski et al gave equimolar doses of PL, PM, and PN orally to five young males [44]. Bioavailability of vitamin B6 was calculated using the area under the vitamin concentration in plasma versus time curve (AUC). The AUC of total vitamin B6 in plasma was larger when PL was adminis­ tered as compared to the PN-dose. The latter resulted in a larger AUC of total vitamin B6 than PM. If the AUC ofPLPin plasma was used as a marker for the effectivity of utilization, PN and PM exhibited a better bioavailability than PL (PN and PM were equal). This indicated a worse availability of PL as compared to PN and PM and corresponds to the higher urinary excretion of 4-PA found after the PL-dose as compared to PN and PM. In studies dealing with the bioavailability of vitamin B 6 , formulation factors have to be taken into consideration . Thak­ ker et al were able to demonstrate pronounced differences in bioavailability of three different formul ations containing 5 mg PN . HCl each in 12 healthy normal weight males [45]. Dosage form s under investigation were common tablets (Stresstabs®, Lederle Laboratories, Pearl River, NY), soft elastic gelatin (SEG) capsules (Aquabiosorb®, RP Scherer North America, St Petersburg, FL) and modified-formulation SEG capsules (mod­ ified by the inclusion of a non-surfactant absorption enhancer to the standard oleaginous vehicle). Concentration-time curves of PL and PLP were observed over a period of 24 hours in plasma. Values obtained for C m ax (data not shown here) and AUC indicated a better bioavailability of PN . HCl ingested using tablets and SEG as compared to modified SEG (Table 2). This observation emphasizes the importance of specifying the preparations used in studies on kinetics . Ethnic differences concerning the bioavailability were observed in the study cited

Table 1. Pharmacokinetic Parameters of Vitamin B6 as Calc ulated from Blood Plasma of Healthy Male Volunteers after Oral

Administration (600 mg Pyridoxine Hydrochloride) and Intravenous Continuous Infusion (100 mg Pyridoxine Hydrochloride

over 6 hours)* e ma,

Oral administration Pyridoxine Pyridoxal 5' -phosphate Pyridoxal 4-Pyridoxic acid Intravenous infusion* * Pyridoxine Pyridoxal 5' -phosphate Pyridoxal 4-Pyridoxic acid

* Means ±

(nmol!L)#

Imax

(hour)

25053 .0 945.3 8682.2 8104.2

± ± ± ±

11 511.4 338.8 1318.5 1384.5

2.3 3.3 3.3

0.913 0.442 0.344 0.331

± 0.140 ± 0.109 ± 0.044 ± 0.045

372.7 892.9 2183.2 1750.3

± ± ± ±

160.8 483.3 494.7 369.5

6.0 9.0 6.2 6.3

6.049 0.268 0.683 0.388

± 0.089 ± 0.227 ± 0.077

1.3

0.029 ± 0.014 0.023 ± 0.015 0.019 ± 0.0l3

± 2.062 0.022 ± 0.021 0.065 ± 0.058 0.024 ± 0.017

SD are reported (n = 9, oral administration; n = 10, intravenous continuous infusion). Data taken from references [35,39].

# Abbreviations used: C m llx = maximal concentration ;

lmax = ti me of maximal concentration ; kw k/3 = rate constants of disposition (fast and slow phase, respective ly).

As the kinetics of pyridoxine obey a one-compartmen t model, the term " K" should be used instead of " ka ".

**

Values of t m ax were calculated starting from the o nset of infusion.

JOURNAL OF THE AMERICAN COLLEGE OF NUTRITION

581

Pharmacokinetics of Vitamin B6 as well [45]. The influence of fonnulation upon bioavailability is underlined by Kaniwa et al [46]. They compared the bio­ availability of 48 different enteric-coated and 5 sugar-coated PLP-containing tablets in 12 males (21 to 52 years of age). Bioavailability was measured by determining urinary excretion of 4-PA over a period of 22 hours. Extent and rate of absorption were strongly dependent upon the formulation of the prepara­ tion, with sugar-coated preparations exhibited the best avail­ ability. Bioavailability dec reased in con'espondence to de­ creased in-vitro solubility of tablets. Pharmacokinetics of continuous intravenous infusion were described in detail [39]: One hundred mg PN . HCI were in­ fused into 10 healthy young males over a period of 6 hours. Concentration-time curves of detectable B6 -metabolites in blood plasma obtained during and following the infusion were analyzed pharmacokinetically. Of special interest are the dif­ fering apparent first-order disposition rate constants of B6 ­ metabolites after oral and intravenous administration (Table 1, both investigators obtained their data by using the same ana­ lytical procedures). The first- pass-effect of liver seems to be of great impact upon metabolism. This would explain the differing utilization of PN . Hel foll owing oral and intravenous admin­ istration. A comparison of AUC of the different metabolites in blood plasma (corrected for dose and elimination rate constant) has shown a significant higher availability followin g the intra­ venous route [39J. For example, the AUC of PLP was 7.5-fold larger after intravenous PN . HCI administration as compared to an oral supplementation .

PHARMACOKINETICS IN ERYTHROCYTES Erythrocytes are ideal for the an alysis of cellular vitamin B6 -metabolism in vivo because of their easy availability. The available information includes both, function al parameters (erythrocyte aspartate aminotransferase, EAST) and kinetic analyses of concentration-ti me curves . Investigations of functional parameters are mostly restricted upon activity of PLP-dependent EAST 2 . Administration of high-dose oral supplements (600 mg PN . HCl) led to a higher enzyme saturation with its coenzyme, but resulted in a lowered maximal activity also [35]. Within 0.7 hours after vitamin­ administration, the activity of EAST + fell significantly from a value of 1236.3 ± 102.8 unitslL to a value of 1124.9 :!:: 181.1 unitslL. Perhaps this was due to alterations in enzyme confor­ mation as a result of build-up of unspecific Schiff's bases. Coefficient of activation (EAST +IEASTo) fell from 1.73 :!::

Table 2. Relative Bioavailability of 5 mg Pyridoxine Hydrochloride from Different Formulations as Analyzed by the Appearance of Pyridoxal and Pyridoxal 5' -Phosphate in Blood Plasma* Pyridoxal

Pyridoxal 5' -phosphate

Aue (nmol' hOllr ' L- 1)# Tablet SEG Mod. SEG

* Means

1175.35 :!: 52 1.89 11 61.81 :!: 47 l.99 987 .42 :!: 461.44* *

1985.70 :!: 1337.92 2075 .45 :!: 1422.87 1598.24 :!: 628.58* *

:+: SO are reported (n = 12). Data taken from reference [45].

= area under the vi tamin concentration in plasma

# Abbreviations used: AVe

versus time curve; SEG = soft elastic gelatin capsule; mod. SEG, modified soft

elastic gelatin capsule (modified by inclusion of a non-surfactant absorption

enhancer).

** Significantl y different

fro m tablet aod SEG (p < 0.05 ).

0.15 to 1.42 :!:: 0.1 5 in the same period of time (not significant). A significant level was reached after 1.3 hours (1.27 :!:: 0.1 2). Concentration-time curves of metabolites in erythrocytes were anal yzed phannacokinetically following an intravenous PN . HCI-infusion [39] . The infusion led to largely increased concentrations of PL and PLP, which were suited for calcula­ tion of kinetic parameters. Pyridoxal reached maximal concen­ trations of 5592.5 :!::1270.5 nmollL; its apparent first-order absorption rate constant was calculated to be 0.597 :!:: 0.154 hour- 1 , the apparent first-order elimination rate constant was 0.200 :!:: 0.024 hour- l PLP reached concentrations of 1646.2 :!:: 400.7 nmollL ; its absorption rate constant was calculated as 0.420 :!:: 0.204 hour- t , the elimination rate constant was 0.479 :!:: 0.238 hour- l Concentrations of other detectable metabo­ lites PN and PMP were too small to become anal yzed pharma­ cokinetically . No PM and 4-PA were detected . The investigations of Anderson et al also indicate a rapid PN-metabolism in erythrocytes [47]. They have injected dif­ ferent doses of PN into a fe male on two occasions (48.6 M-mol und 118 M-mol PN). In both, blood plasma and erythrocytes, a rapid elimination of PN was observed. Simultaneously, a large increase in concentrations of aldehyde forms PL and PLP OCCUlTed. PN was eliminated faster fro m erythrocytes than could be explained by the synthesis of PL and PLP. As non­ phosphorylated forms of vitamin B6 are able to cross the erythrocyte membrane readily [48], a rapid exchange between erythrocytes and blood plasma is possible . The distribution of PL between erythrocytes and blood plasma is at least in part caused by the different affinity to its binding proteins hemo­ globin and albumin [49]. The synthesis of the coenzyme form PLP in erythrocytes and tissues depends on the sufficient supply of other mic­ ronutrients. Flavin mononucleotide serves as a coenzyme for

Erythrocyte aspartate aminotransferase (EAST) activity is a common parameter for j udgement of vitamin E6 status . Enzyme acti vity is measured in erythrocyte hemolysates and yields the follo wing parameters: EASTo, if coenzyme PLP was not added to the assay (basal enzy me ac tivi ty ); EAST +. excess PLP was added to the hemolysate (maximal enzyme activity); activation coefficient EAST +fEASTo. indicative of enzyme saturation with its coenzyme.

2

582

VOL. 14, NO.6

Pharmacokinetics of Vitamin B6 pyridox amine phosphate oxidase [25,50]. Therefore, the syn­ thesis of PLP depends on an adequate vitamin B2 status. Indeed a correlation between parameters of ribofl avin and vitamin B2 status among 473 elderly males and fem ales was observed in the Dutch Nutrition Survei llance System [5 1] . Pyridoxal ki ­ nase, the second enzyme involved in PLP synthesis, is acti vated by Zn + + and Mg" + [26,28]. An impact of deficie ncy in these micron utrients on vitamin B6 utilization can not be excluded. Vitamin B6 status itself has an impact on the vitamin ' s kinetics . Lui et al demonstrated a decrease in systemic clearance of PLP in four male subjects (23 to 30 years of age) after an intrave­ nous supplemen tati on with 122 [Lmol PN· HCl/day over 4 weeks [52]. The clearance decreased significantly from 69.0 :+:: 14.2 mJ/min ute to 33.4 :+:: 9.3 ml/minute. The pharmacokinetic studies cited in the present review excluded defici ency states by measurement of baseline levels of PLP in plasma or 4-PA excretion in urine [42,43,45]. In other trials, vitamin supple­ ments were gi ven before administration of the test dose [35,39,53].

amounted to only 10% that observed in urine [57]. When they perfused rat livers, 3% of radioactivity was recovered in bile fluid within 4 hours. Main metabolites were PLP, PL, and PN. Heard and Annison have demonstrated in their animal model (chic ken), that only 1% of a usual daily vitami n B6 reset uptake is recycled enterohepatically [58]. They have not differentiated for the metabolites of vi tamin B6 , but analyzed them in total using the Saccharomyces uvarum assay. Especially if the nearly quanti tative absorption of vitamin B6 is considered [35], ex­ cretion via bile seems to be negligi ble. Renal clearances were calculated under the steady-state­ conditions of a continuous PN . HCl infusion (Table 3) [53]. According to these results there is a net reabsorption of PL in renal tubulus, whereas 4-PA and PN are secreted. In this investigation, the apparent first-order constant for renal excre­ tion also was calculated for the metabolites (Table 3). Only for 4-PA, a second deep compartment was feasible. The rate con­ stant obtained for PN is much lower than the one calcul ated from plasma (see above). This is because as the latter is a systemic constant which refl ects the sum of all elimination processes of PN (metabolism and excretion).

STORAGE OF VITAMIN B6 Muscle phosphorylase functions as a repository for vi tamin B 6 , phosphory lase contains stoichiometric amounts of PLP [54]. The total body pool of vi tamin B6 was estimated to be 1000 [Lmol in humans [55]. The data were calculated from muscle biopsies in fi ve females (1 9 to 58 years of age) and seven males (24 to 49 years of age). Females exhibited sligh tly hi gher vitam in B6 concentrations in muscle tissue as compared to males.

KINETICS OF EXCRETION Renal eliminati on is the main pathway of excretion of vitamin B 6 . It has been shown that 73.6 :+:: 7.2% of an intra­ venous PN dose (100 mg PN . HCl) was excreted in urine in form of different metabolites [53]. The largest part of this was 4-PA which constituted 63.7 :+:: 6.4% of the dose infused . PN (6.7 :+:: 1.8%) an d PL (3 .3 :+:: 0.9%) were also detectable. The difference between uri nary excretion and dose infused (approx­ imately 74% recovery) was explained by the excretion of not-detected metabolites such as 4-pyridoxic acid 5' -phosphate [56], an enhanced protein-binding of vitamin B6 in tissues caused by the high dose administered - as described for glyco­ gen phosphorylase [54] - and alternative routes of elimination. Similar urinary excretion patterns were observed by Ubbink et al in nine healthy females, who received oral doses of up to 100 mg PN . HCI [43]. Non-renal routes of vitamin B6 excreti on are poorly studied. At the moment we depend on data obtained from ani mal studies for this purpose. Following an injection of 14C_PN into rats, Lui et al found an excretion of radioactivity in bile fluid which

JOURNA L OF THE AMERIC AN COLLEGE OF NUTRITION

AGE AND SEX DEPENDENCY OF METABOLISM Data indicated that metabolism of vitamin B6 is strongly altered in premature infants. Raiten et al intravenously admin­ istered PN to premature infants « 30 weeks gestation) and failed to observe any raise of PLP in serum [59], which is contradictory to the results described for healthy adults. The dose administered was 1.97 or 3.94 [Lmol, depending on in­ fants' body weight. In erythrocytes of these infants, a pro­ nounced increase of PLP concentration was observed which indicated a completely independent PLP-pool in these cells. The response of serum PLP was also negligible when admin­ istered in an equimolar dose of PL in travenously to another group of infants « 28 weeks gestation). When these findi ngs Table 3. First-Order Rate Constants for Renal Excretion and Renal Clearance of Vitamin B6 Metabolites, as Obtained from Continuous Intravenous Infusion of 100 mg Pyridoxine Hydrochloride into Healthy Male Subjects*

CI, Metabol ite

(ml/minlltel 1.73 m 2 )#, **

k" (hollr- I)

k/3 (hour- I)

PN

25 7.5 ± 92.1 20.1 ± 6.0 249.8 ± 48.2

1. 231 ± 0.877 0.653 ± 0.360 0.314 ± 0.118

0.048 ± 0.070

PL 4-PA

* Means

:±: SO are reported (n

=

10). Data take n from refe rence (53].

k~, apparent first-order rate con stants for renal excretion (fast and slow phase, respectively) .

# Abbrevi ations used: CI" renal clearance; k o ,

** The binding of pyridoxal to plasma-p roteins was not take n into considerat ion for the calculation of its renal clearance.

583

Pharmacokinetics of Vitamin 8 6 were compared to full-term newborns, the latter utili zed par­ enterally PN more effectively. Reinken et al injected PN intra­ muscularly to newborns and observed an increase in urinary excretion of 4-P A [60]. In contrast to earlier observations, recent investigations have demonstrated that vitamin B6 metabolism is unaltered in elderly individuals. Ferroli and Trumbo have shown that plasma PLP concentration, EAST activity, and plasma alkaline phosphatase activity were not different between two age groups of white males (20 to 30 vs. 60 to 70 years of age) [61). Urinary 4-PA excretion after an oral test dose of [2H)pyridoxine was similar in both age groups. Controlled clinical studies on sex differences in human vitamin B6 metabolism are missing. Data from animal studies suggested pronounced sex-dependent differences in tissue lev­ els of enzymes of vitamin B6 metabolism, such as pyridoxal kinase, PMP oxidase, and PLP hydrolase [50,62]. A higher absolute and percentual urinary 4-PA excretion was found in male rats compared to their female counterparts after oral PN administration [62).

CONCLUSIONS There are some data available now on the pharmacokinetics of vitamin B6 in healthy subjects. Unfortunately our knowledge is limited to the data from male volunteers. Kinetic data from females are rare. Ethnic differences were neglected until today. Another insufficiency is the lack of data in disease states. Additional efforts are necessary to extend our understanding of the pharmacokinetics in such cases. At the moment our knowl­ edge about B 6 -kinetics in critically ill patients stems mostly from patients with liver disease and alcoholism rreview in reference 63]. These studies indicate pronounced differences in the metabolism of vitamin B6 in cases of disease which justify further exploration.

REFERENCES 1. Bender DA: Vitamin B6 requirements and recommendations. Eur J Clin Nutr 43:289-309, 1989. 2. Kang-Yoon SA, Kirksey A, Giacoia G, West K: Vitamin B-6 status of breast-fed neonates: influence of pyridoxine supplemen­ tation on mothers and neonates. Am J Clin Nutr 56:548-558, 1992. 3. Newman V, Lyon RB , Anderson PO: Evaluation of prenatal vita­ min-minerai supplements. Clin Pharm 6:770- 777, 1987. 4. Bassler KH : Megavitamin therapy with pyridoxine. Int J Vit Nutr Res 58:105- 118, 1988. 5. Boers GHJ, Sma1s AGH, Trijbels FJM, Fowler B, Bakkeren JAJM, Schoonderwaldt HC, Kleijer WJ, Kloppenborg PWC: Heterozy­ gosity for homocystinuria in premature peripheral and cerebral occlusive arterial disease. N Engl J Med 313:709- 715, 1985. 6. Brattstrom L, Israelsson B, Norrving B, Bergqvist 0 , Thome J, Hultberg B, Hamfelt A: Impaired homocy steine metabolism in

584

early-onset cerebral and peripheral occlusive arterial disease. Ef­ fects of pyridoxine and folic acid treatment. Atherosclerosis 81: 51-60, 1990. 7. National Research Council (ed): "Recommended Dietary Allow­ ances," 10th ed. Washington , DC: National Academy Press, pp 142-150, 1989. 8. Bundesverband der phannazeutischen Industrie (ed) : "Rote Li ste 1992." Aulendorf, Gennany: Editio Cantor, 1992. 9. Labadarios 0 , Rossouw .IE, McConnell JB , Davis M, Williams R: Vitamin B" deficiency in chronic liver disease- evidence for in­ creased degradation of pyridoxal-5' -phosphate. Gut 18: 23-27, 1977. 10. Lumeng L, Li T-K: Vitamin B6 metabolism in chron ic alcohol abuse. Pyridoxal phosphate levels in plasma and the effects of acetaldehyde on pyridoxal phosphate synthesis and degradation in human erythrocytes. J Clin Invest 53: 693- 704, 1974. 11. Veitch RL, Lumeng L, Li T-K: Vitamin B6 metaboli sm in chronic alcohol abu se. The effect of ethanol oxidation on hepatic pyridox al 5' -phosphate metaboli sm. J Clin Invest 55 : 1026-1 03 2, 1975. 12. Kleiner MJ, Tate SS , Sullivan .IF, Chami J: Vitamin B6 deficiency in maintenance dialysis patients: metabolic effects of repletion. Am J Clin Nutr 33:1612-1619, 1980. 13. Teehan BP, Smith LJ, Sigler MH, Gilgore GS, Schleifer CR: Plasma pyridoxal-5' -phosphate levels and clinical correlations in chronic he­ modialysis patients. Am .I Clin Nutr 31: 1932-1936, 1978. 14. Kriley M, Warady BA: Vitamin status of pediatric patients receiv­ ing long-term peritoneal dialysis. Am .I Clin Nutr 53:1479, 1991. 15. Koplan JP, Annest JL. Layde PM, Rubin GL: Nu trient intake and supplementation in the United States (NHANES II). Am J Public Health 76:287-289, 1986. 16. Stewart ML, McDonald JT, Schucker RE, Henderson DP: Vita­ min/mineral supplement use: A telephone survey of adults in the United States. J Am Diet Assoc 85 : 1585-1590, 1985. 17. Medeiros OM, Bock MA, Ortiz M, Raab C, Read M, Schutz HG, Sheehan ET, Williams OK: Vitamin and mineral supplementation practices of adults in seven western states. J Am Diet Assoc 89:383- 386, 1989. 18. Schaumburg H, Kaplan J, Windebank A, Vick N, Rasmus S, Pleasure 0 , Brown MJ: Sensory neuropathy from pyri doxine abuse. A new megavitamin syndrome. N Engl J Med 309:445-448, 1983. 19. Nisar M, Watkin SW, Bucknall RC, Agner RAL: Exacerbation of isoniazid induced peripheral neuropathy by pyridoxine. Thorax 45 :419-420, ] 990. 20. Parry GJ, Bredesen DE: Sensory neuropathy with low-dose pyri­ doxine. Neurology 35 :1 466-1468, 1985. 21. Chrisley BM, McNair HM, Driskell JA: Separation and quan tifi­ cation of the B6 vitamers in plasma and 4-pyridoxic acid in urine of adol escent girl s by reversed-phase high-performance liquid chromatography. J Chromatogr 563:369-378, 1991. 22. Sampson DA, O'Connor OK: Analysis of B-6 vitamers and pyri­ doxic acid in plasma. tissues and urine using high perfomlance liquid chromatography. Nulr Res 9: 259- 272, 1989. 23. Sharma SK, Dakshinamurti K: Determination of vitamin B6 vita­ mers and pyridox ic acid in biological samples. J Chromatogr 578:45- 51 , 1992. 24. Shephard GS, Louw MEl, Labadarios 0 : Analy sis of vitamin B6

VOL. l4, NO.6

­

Pharmacokinetics () ' Vitamin B6 vi tamers in plasma by cation-exchange high-performance li quid chrom atography. J Chromatogr 416:138-143, 1987. 25. Clements JE, Anderson BB: Pyridoxine (pyridoxami ne) phosphatc oxidase activity in the red cell. Biochim Biophys Acta 613:40 1­ 409, 1980. 26. Hamfelt A: Pyridoxal kinase activity in blood cells. Clin Chim Acta 16:7-1 8, 1967. 27. Lumeng L, Li T-K: Characterization of the pyridoxal 5'-phosphate an d pyridoxamine 5' -phosphate hydrolase activity in rat liver. Identity with alkaline phosphatase. J Bioi Chern 250:8 126-8131 , 1975. 28. McCormick DB, Gregory ME, Snell EE: Pyridoxal phosphoki­ nases. I. Assay, distribution, purification, and properties. J BioI Chem 236:2076-2084, 1961. 29. Men-ill AH, Henderson JM, Wang E, McDonald BW, Mi llikan WJ: Metabolism of vitamin B-6 by human liver. J Nutr 114:1664­ 1674, 1984. 30. Stanulovic M, Jeremic V, Leskovac V, Chaykin S: New pathway of conversion of pyridoxal to 4-pyridoxic ac id. Enzyme 21:357­ 369, 1976. 3 1. Dost FH: "Grundlagen der Pharmakokinetik," 2nd ed. Stuttgart: Thieme Verlag, pp XVII, 1968. 32. Leinert J, Simon I, Hotzel D: Methoden und deren Wertung zur Bcstimmung des Vitamin-B 6 -Versorgungszustandes beim Men­ schen. 6. Mitteilung. S-PLP. Die Aussagefahi gkeit des Parameters. Int J Vit Nutr Res 53:166- 178,1983. 33. Mascher H: Determination of total pyridoxal in human plasma fo ll owing oral administration of vitamin B6 by hi gh-performance liq uid chromatography with post-column deri vatization. J Pharm Sci 82:972-974, 1993. 34. Forstner C: "4-Pyridoxinsaure-Ausscheidung im Urin unter und nach langfristig hochdosierten Vitamin-B 6 -Gaben an gesunden Erwachsenen" (Doctoral Thesis). Germany, University Giessen, 1990. 35. Speitling A: "Wirkungen akut und chronisch hochdo sierter Vita­ min B6-Gaben. Biokinctische Untersuchungen am Menscben." Niederkleen, Germany: Wissenschaftlicher l'achverlag Dr. Fleck, 1991. 36. Gladtke E, von Hattingberg HM: "Pbarmakoki netik," 2nd ed. Berlin: Springer Verlag, pp 17-44, 1977. 37. Booth CC, Brain MC: The absorption of tritium-labelled pyridox­ ine hydrochloride in the rat. J Physiol 164:282-294, 1962. 38. Lumeng L, Lui A, Li T-K: Plasma content of B6 vi tamers and its relationship to hepatic vitamin B6 metabolism. J Clin Invest 66: 688-695, 1980. 39. Zempleni J, Kubler W: The utili zation of intravenously infused pyridoxine in humans. Clin Chim Acta 229:27- 36, 1994. 40. Anderson BB, Newmark PA, Rawlins M, Green R: Plas ma bi nding of vitamin B6 compounds. Nature 250:502-504, 1974. 41. Whyte MP, Mahuren JD, Vrabel LA , Coburn SP: Markedl y in­ creased circulating pyridoxal-5' -phosphate levels in hypophos­ phatasia. J Cl in Invest 76:752- 756, 1985. 42. Edwards P, Liu PKS, Rose GA: Liquid chromatograph ic studies of vitamin B6 metabolism in man. Clin Chim Acta 190:67-80, 1990. 43. Ubbink lB, Serfontein WJ, Becker PJ, de Villiers LS : Effect of different levels of oral pyridoxine supplementation on plasma

JOURNAL OF THE AMERICAN COLLEGE OF NUTRITION

pyridoxal-5 ' -phosphate and pyridoxal levels and urinary vitamin B-6 excret ion. Am J Clin Nu tr 46:78-85, 1987 . 44. Wozenski JR, Leklem JE, Miller LT: The metabolism of small doses of vitamin B-6 in men. J Nutr 110:275-285, 1980. 45. Thakker KM, Sitren HS, Gregory JF Ill, Schmidt GL, Baumgartner TG: Dosage fornl and formulation effects on the bioavailability of vitamin E, riboflavin, and vitamin B-6 from multivitamin prepa­ rations. Am J Clin Nutr 45: 1472- 1479, 1987 . 46. Kani wa N, Ogata H, Aoyagi N, Keibuchi M, Shibaki T, Ejima A, Takanashi S, Kamiyama H, Suzuki H, Hinohara Y: Bioavailability of pyridoxal phosphate from enteric-coated tablets. I. Apparent critical dissolution pH and bioavai1ability of commercial products in humans. Chem Pharm Bull 33:4045-4049, 1985. 47. Anderson BB, Pen'y GM, Clements JE, Greany MF: Rapid uptake and clearance of pyridoxine by red blood cells in vivo. Am J Clin Nutr 50:1059-1063, 1989. 48. Mehansho H, Henderson LM: Transport and accumulation of pyridoxine and pyridoxal by erythrocytes. J Bioi Chem 255: 11901-11907, 1980. 49. Ink SL, Henderson LM: Vitamin B6 metabolism. Ann Rev Nutr 4:455-470, 1984. 50. Rasmussen KM , Barsa PM, McCormi ck DB , Roe DA: Effect of strain, sex and dietary ribofl avin on pyridoxine 5' -phosphate oxi ­ dase activi ty in rat tissues. J Nutr 110:1940-1946, 1980. 51. Lowik MRH , van den Berg H, Kistemaker C, Brants HAM, Brus­ saard JH: Interrelationships between riboflavin and vitamin E6 among el derl y people (Dutch Nutrition Surveillance System). Int J Vit Nulr Res 64: 198-203, 1994. 52. Lui A, Lumeng L, Aronoff GR: Relationships between body store of vitamin Bo and plasma pyridoxal-P clearance: metabolic balance studies in humans . J Lab Clin Med 106:491-497, 1985. 53. Zempleni J, KUbler W. Metabolism of vitamin B6 by human kidney. Nutr Res 15:187-1 92, 1995. 54. Black AL, Guirard BM, Snell EE: Muscle phosphorylase as a reservoir for vitam in B-6 in rats. J Nutr 107:1962- 1968,1977. 55 . Coburn SP, Lewi s DLN, Fink WJ, Mahuren JD, Schaltenbrand WE, Cost ill DL: Human vitamin B-6 pools estimated through muscle biopsies. Am J Clin Nutr 48:291 - 294, 1988. 56. Contractor SF, Shane B: 4-Pyridoxic acid 5 '-phosphate: a new metabolite of pyri doxal in the rat. Biochem Biophys Res Commun 39:11 75-1181 , 1970. 57. Lui A, Lumeng L, Li T-K: Biliary excretion of 14C-Iabeled vita­ min B-6 in rats. J Nutr 11 3:893-898, 1983. 58. Heard GS, An nison EF: Gastrointestinal absorption of vitamin B-6 in the chick en (Gallus domesticu s) . J Nutr 116: 107-120, 1986. 59. Raiten DJ, Reynolds RD, Andon MS, Robbin s ST. Fletcher AB: Vitamin B-6 metabolism in premature infants. Am J Clin Nutr 53:78-83, 1991. 60. Reinken L, Berger H, Dapunt 0, Komer WF: Beziehungen zwis­ chen der Vitamin B6-Versorgung Schwangerer und der Vitamin B6-A usstattung ihrer Neugeborenen. Z Kinderhei lk 115:163- 174, 1973. 61. Ferroli CE, Tru mbo PR: Bioavai lability of vitamin B-6 in young and older men. Am J Cli n NutI' 60:68-7 1, 1994 . 62. Bode W, Mocking JAJ, van den Berg H: Influence of age and sex on vitamin B-6 vitamer di stribution and on vitamin B-6 metabo­ lizing enzymes in Wi star rats. J Nutr 121:318-329,1991.

585

Pharmacokinetics of Vitamin B6 63. McCormick DB, Munro HN: Liver in relation to water-soluble vitamins. In Arias IM, Boyer JL, Jakoby WB , Schachter DA, Shafritz DA (eds) : "The Liver: Biology and Pathobiology." New Yark: Raven Press Ltd., pp 563-583, 1994. 64. Gibaldi M, Perrier D: "Pharmacokinetics," 2nd ed. New York: Marcel Dekker, Inc., 1982.

APPENDIX Concentration-time curves of metabolites in blood plasma are described by the following equations [31,36,64]. Equations 1 and 2 describe the elimination of intravenously injected substances assuming an one-compartment model or a two­ compartment model, respectively. Equations 3 and 4 are valid during (eq. 3) and following (eq. 4) a continuous intravenous infusion. Equation 5 (Bateman's function) describes the curve following oral administration if linear kinetics are allowed to be assumed.

586

(1) (2)

C = (v/(V . K)) . (1 - e- Kt ) C

=

(v/(V . K . T)) . (e- KT

-

(3 )

1) . e- Kt

(4)

where A, B, Co = vitamin concentrations in plasma (erythro­ cytes) immediately after injection (zero-time intercepts; ob­ tained by extrapolation), C = vitamin concentratio n at time t, K = apparent first-order elimination rate constant, ka = apparent first-order absorption rate constant, k", = first-order fast dispo­ sition rate constant, kf3 = first -order slow disposition rate constant, t = time, T = constant dosing interval (time during which infusion took place), v = rate of vitamin infusion, V = apparent volume of distribution.

Received September 1994; revision accepted May 1995.

VOL. 14, NO.6