Abstract-The effect of selected antacids on the amount ofFK 506 in solution in simulated gastric juice has been studied. FK 506 (2·5 mg) was incubated in 100 mL simulated gastric fluid (SGF) with the equivalent of 500 mg of various antacids. The addition of Mylanta and Turns resulted in 14 and 30&apos;1., loss of FK 506, respectively, in 24 h; 98% loss was observed in 12 h in the presence of Mag-Ox; 100&apos;Yo loss was observed in the presence of magnesium oxide powderin 2 h. The loss ofFK 506 from these solutions appears to be due to a pH mediated degradation of FK 506. The addition of aluminium hydroxide gel USP (Roxane) to the FK 506 solution resulted in a 35°;;, loss within 2 min but no further loss was noted for 24 h. indicative of adsorption ofFK 506. These results suggest that until additional in-vivo studies are carried out. it is prudent not to dose FK 506 and antacids at the same time to avoid potential interactions

A K Abu-Elmagd

F Jain

G J Venkataramanan

K Ptachcinski

M Steeves

R J Burckart

S Fung

Starzl

T E To

Y Abdallah R Ii

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1. Pharm. Pharmacol. 1991, 43: 574- 577 
Received November 30. 1990 
«:: 19911. Pharm. Pharmacol. 
In-vitro Interaction of a Novel Immunosuppressant, 
FK 506, and Antacids 
M. STEEVES. II. Y. ABDALLAH. R. VENKATARAMANAN. G. J. BURCKART. R. J. PTACHCINSKI. K. ABU-ELMAGD. 
A. K. JAIN, F. FUNG. S. TO DO AND T. E. STARZL 
Schools of Pharmacy and Medicine. University 0/ Pittsburgh. Pittsburgh. Pennsvimnia 15261. USA 
Abstract-The effect of selected antacids on the amount ofFK 506 in solution in simulated gastric juice has 
been studied. FK 506 (2·5 mg) was incubated in 100 mL simulated gastric fluid (SGF) with the equivalent of 
500 mg of various antacids. The addition of Mylanta and Turns resulted in 14 and 30'1., loss of FK 506, 
respectively, in 24 h; 98% loss was observed in 12 h in the presence of Mag-Ox; 100'Yo loss was observed in 
the presence of magnesium oxide powderin 2 h. The loss ofFK 506 from these solutions appears to be due to 
a pH mediated degradation of FK 506. The addition of aluminium hydroxide gel USP (Roxane) to the FK 
506 solution resulted in a 35°;;, loss within 2 min but no further loss was noted for 24 h. indicative of 
adsorption ofFK 506. These results suggest that until additional in-vivo studies are carried out. it is prudent 
not to dose FK 506 and antacids at the same time to avoid potential interactions. 
FK 506 (Fig. I) is a macrolide with a molecular weight of 822 
isolated from the cultures of the fungus Streptomrccs 
tsukuhacllsis (Sawada et al 1987). It is highly lipophilic. 
poorly soluble in water and alkanes. and freely soluble in 
ether, chloroform and ethyl acetate (Tanaka et al 1987). FK 
506 is nearly 400 times more potent than cyclosporin (CsA) 
in inhibiting lymphocyte proliferation in mixed lymphocyte 
cultures (Zeevi et al 1987). The drug has been shown to 
reverse or prevent the rejection of heart. liver. kidney. 
pancreas, lung. intestine and skin grafts in mice, rats. dogs, 
monkeys and bab00ns (Fung et a! ! 990; T odo et a! 1990). FK 
506 is currently undergoing clinical investigations at the 
University of Pittsburgh. Initial results indicate FK 506 to 
provide better immunosuppression in liver transplant reci-
pients than CsA (Todo et al 1991). 
FK 506 is administered either orally or intravenously (i. \.J 
to transplant patients. Following oral administration. FK 
506 is poorly and incompletely absorbed. with low bioavaila-
bility of FK 506 in transplant patients. Absorption is also 
highly variable among patients (Venkataramanan et al 
1990). The large variability in the oral absorption of FK 506 
MeO Me 
o 
.J 
ME' 
MeO 
FIG. I. Chemica I structure or FK SOil. 
Correspondence: R. Venkataramanan. 71 ~ Salk HaIL School or 
Pharmacy. Uni\crsity ofPittshurgh. Pittshurgh. PA 15261. USA. 
may be related to physiological factors associated with the 
transplant patients. formulation factors. or interactIOns with 
other drugs. Transplant patients routinely receive a large 
number of concurrent medications including prophylactic 
antacids. Drug interactions with antacids are well docu-
mented (Hansten & Horn 1989; American Society ofHospi-
tal Pharmacists 1990). Antacid-mediated alterations in the 
extent of absorption of FK 506 may lead to the rejection of 
the transplanted organ or FK 506-related toxicity. This 
study was designed to determine if representative antacid 
preparations 01 their active ingrediems chemicaliy or physi-
cally interact with FK 506 in-vitro. 
Materials and Methods 
Materials 
Table I describes the products and chemicals used in this 
study. Chemicals were analytical grade and were purchased 
from Fisher Scientific. Pittsburgh. PA. Antacids werc 
sclected on the basis of their use in transplant paticnts at our 
institution. Also included in this study is Mag-Ox tablets. 
commonly given to transplant patients for magnesiulll 
replacement. Simulated gastric fluid was prepared without 
pepsin (USP 1980). 
Study design 
Simulated gastric fluid (SG F) without pepsin. 100 mL. was 
warmed and maintained at 37 C in a Waters (Division of 
Millipore Corp .. Milford. MA.) isotcmp !nclIbator. FK S06. 
0·25 mL of a 10 mg mL -I solution (FK 5061.\. injection. 
Fujisawa Pharmaceuticals. Kyoto. Japan) \\as added to the 
SG F to achieve a concentration of 25 Ilg m L The 
equivalent of 500 mg of the antaCId in the form 01 liquid 
formulation. crushed tablet. or chemical was then added and 
mixcd. Samples (I mL) \\ere takcn before and at 2. 5. 10. 15. 
20. 30 min. I. I· 5. 2. 12 and 24 h after the addil \l1T1 of I he lcst 
substance. Samples were acidified with 0·2 mL or I M Hel 
immediatel\ after collection. Previous studies have sho\\11 
that FK 506 is stable for at least a week under the\c 
conditions IAbdallah et al 1990). The samples were cenln-
IN-VITRO INTERACTION OF FK S06 AND ANTACIDS 575 
Table I. Summary of products and chemicals. 
pH' 
Amount 
tested SGF SGF 
Product/chemical Active ingredient(s) in 100 mL Water initial 24 h 
Simula ted gastric fluid lOOmL J.3 1·3 1·3 
FK 506 for injection 10 mgmL- 1 2'5mg 5·7 1·4 \·3 
Aluminium hydroxide 500mg 6·2 \·5 1·3 
Aluminium hydroxide gel 500mg 4·8 3·5 H 
dried 
Aluminium hydroxide gel AI(OH») gel 450mg/5 mL 5'5mL 6·7 3·3 3·5 
USP (Roxane) 
Amphogel suspension AI(OHh gel 320 mgj5 mL 8mL 6·6 H 3·6 
Calcium carbonate 500mg 9·4 5·\ 6·9 
Turns CaCO) 500mg I tab 4·8 4·6 7·1 
Magnesium chloride 500mg 5·5 1-4 \·3 
Magnesium oxide 500mg 9·6 9·6 9·1 
Mag-Ox Mg 400mg \ tab 10·5 6·0 9·3 
Maalox suspension A\(OH)) gel 225 mg/5 mL II mL 7-8 6-4 6·5 
Mg(OHh 200mg/5 mL 
Mylanta suspension Al(OH)] gel 200 mg/5 mL 12'5mL 7·9 3-9 7·1 
Mg(OHh 200mg/5 mL 
'pH of water or simulated gastric fluid after the addition of liquid antacid, dry powder or crushed tablets. 
fuged at 4000 g for 10 min. FK 506 in the sample was 
measured by HPLC. 
Assay 
A sample of the clear supernatant (100 ilL) was injected, with 
a WISP 712 automatic sample injector (Waters Associates, 
Milford, MA), onto a Supelco C-18, S 11, 4·6 x 150 mm 
column maintained at 70C with a Waters TCM heater. The 
mobile phase consisting of 70% acetonitrile in water was 
pumped through the column at a flow rate of 1·2 mL min-I. 
The column effluent was monitored at 214 nm with a Waters 
440 detector. Under these conditions, the retention time for 
FK 506 was about S min. Peaks were integrated using a 
Hewlett Packard model 3390A integrator. Injections were 
made in triplicate and each experiment was repeated twice. 
Mean peak area of FK 506 was compared with the value 
obtained for the zero time sample. All samples were analysed 
within 48 h of collection. The calculations were corrected for 
the changes in volume when liquid antacids were tested. The 
specificity of the assay was tested by intentionally degrading 
FK S06 under acidic (6 M HCl) and basic (pH 10) conditions. 
The pH of the SGF was measured immediately and 24 h 
after the addition of the test product (Tablc I). An additional 
pH measurement was made immediately after adding the test 
substance to 100 mL of water. 
Statistical methods 
Linear regression was used to calculate the slopes of the 
mean percentage of FK 506 remaining vs time curves. An F 
test was used to determine if the slopes were significantly 
different from zero. A difference was considered to be 
significant for P s O·OS. 
Results and Discussion 
Fig. 2 shows chromatograms of (A) pure FK 506 solution in 
mobile phase; (8) FK 506 in simulated gastric juice; (C) a 
samplc of FK 506 that was intentionally degraded with 6 M 
HCl and elevated temperature (60'C) and (D) FK 506 
degraded at pH 10. The absence of any peaks at the retention 
time corresponding to FK 506 in Fig. 2C and D indicates that 
the assay is specific for unchanged FK 506. The minimum 
limit of detection was 10 ng of FK 506 injected on the 
column, with a signal to noise ratio of 5 to I. The standard 
curve was linear over a range of 10 to 3000 ng of FK 506 
injected on the column. The slope of the calibration curve 
was (mean±s.d.) 0'27S±0'0IS, the intercept 1·5±2·9 and 
the correlation coefficient 0·993 or more. The intra-day 
coefficient of variation for injections (n = 6) ranging from 12 
to 1200 ng was no more than 3%. The inter-day coefficient of 
variation was no more than 8 'Yo for injections (n = 6) ranging 
from 400 to 1200 ng. The percent deviation of estimated 
concentration from actual concentration did not exceed 10'/'(, 
over a range of 100 to 1500 ng injected onto the column. 
The percentage of FK 506 remaining in solution as a 
function of time in the presence of various antacids is 
illustrated in Figs 3 and 4. Of the products and chemicals 
tested, aluminium hydroxide powder, aluminium hydroxide 
gel dried, Amphogel, calcium carbonate, Maalox, and 
magnesium chloride did not affect the concentration of FK 
S06 over 24 h. The slopes of the percent remaining vs time 
curve in the presence of these compounds were not signifi-
cantly different from the control slope. 
Significant loss of FK 506 from solution was observed by 
2 h in the presence of aluminium hydroxide gel USP 
(Roxane, Fig. 4) or magnesium oxide powder. With alumi-
nium hydroxide gel USP (Roxane), there was nearly 35% FK 
506 loss within 2 min; the slope of this portion of the curve 
was 20'5, compared with a control slope of 0·004. Interest-
ingly, no further loss ofFK 506 was seen for the remainder of 
the 24 h. FK 506 was completely lost in 2 h in the presence of 
magnesium oxide powder (Fig. 3). This loss appears to 
follow a first-order profile, with a reaction rate constant of 
0·052 min -1. Significant decrease in the percent FK 506 
remaining in solution was also observed between 2 and 24 h 
for Mag-Ox, Tums and Mylanta (Slopes -0'136, -0'015 
and -0'009, respectively; Fig. 4). 
Most of our observations can be explained by considering 
576 M. STEEVES ET AL 
FK506 
FK506 A B c D 
o 2 4 6 8 10 o 2 4 6 8 10 o 2 4 6 8 10 o 2 4 6 8 10 
Time (min) 
FIG. 2. Chromatograms ofFK 506 solutions; (A) pure FK 506 solution in methanol (5 Jlg mL -I); (8) FK 506 solution 
in simulated gastric fluid (25 Jlg mL -I); (e) FK 506 solution degraded by acid and heat and (D) FK 506 degraded at 
pH 10. 
120 
Ol 80 c 
'c 
140'\~. ~ u. 
o -L~~~~~~=~==~&-~'---6~~ ~~~~ o 60 120 720 1440 
Time (min) 
FIG. 3. Mean % FK 506 remaining vs time over 24 h (0) FK 506 
control SGF solution; and FK 506 SGF solution with (A) alumi-
nium hydroxide; C-) aluminium hydroxide dried gel; (0) calcium 
carbonate; ce) magnesium chloride; and ("') magnesium oxide. 
120 
~ • 
60 120 720 1440 
Time (min) 
FIG. 4. Mean 0;', FK 506 remaining l'S time over 24 h for (0) FK 506 
control SGF solution; and FK 506 SGF solution with (A) alumi-
nium hydroxide gel USP (Roxane); (0) Amphogel; (_) Mag-Ox; (e) 
Turns; ("') Mylanta; ('7) Maalox. 
the effect of pH on FK 506 stability. FK 506 was found to be 
unstable under basic conditions; more than 30%, of the drug 
is lost in 24 h at pH 7-4 (37 c C), while total decomposition 
occurs in 2 h at pH 10 (25"C). At pH values below 7, no 
appreciable loss was observed in 24 h (Abdallah et al 1990). 
The acidification of samples immediately after collection 
prevented any potential change in FK 506 concentration in 
the solution with time. The pH of the FK 506 solution in the 
presence of aluminium hydroxide powder, aluminium hyd-
roxide gel dried, Amphogel, or magnesium chloride was 3·6 
or less while in the presence of Maalox, the pH was 6·5. 
Correspondingly, there was no loss of FK 506 from the 
solution when these compounds were present. 
The pH of FK 506 solution in the presence of calcium 
carbonate also increased over time (5·1 to 6·9); however, thc 
5% loss FK 506 over 24 h was not significantly different from 
control values. The pH ofFK 506 solution in the presence of 
Mylanta changed from 3·9 to 7·2 over 24 h. No significant 
loss ofFK 506 was observed at 12 h; however, after 24 h 14% 
ofFK 506 was lost (Fig. 4). The pH ofFK 506 solution in the 
presence ofTums increased over time (4,6-7·1) with the slow 
dissolution of crushed powder into thc SGF. This may 
explain the 30% loss of FK 506 seen between 2 and 24 h 
(Fig. 4). 
Magnesium oxide (MgO) in SGF had an initial pH above 9 
and remained so over 24 h. The high pH may explain the 
significant degradation of FK 506 (loss of 100'Yo in 2 h) in the 
presence of MgO (Fig. 3). Mag-Ox tablet, when crushed to a 
fine powder, did not evenly disperse throughout the SGF. 
This may explain the slow rise in pH (6-9' 3) from 5 min to 24 h 
as the powder slowly wetted and dissolved and the slower 
rate of loss of FK 506 (Fig. 4). A 98'1" loss of FK 506 was 
observed after 12 h. Conversely, magnesium chloride, which 
is acidic, did not have any significant effect on FK 506 (Fig . 
3). This observation supports the role of pH on the stability 
of FK 506 rather than a specific interaction between the 
compounds. 
The percent remaining vs time profile obtained with 
aluminum hydroxide gel USP (Roxane) was differcnt from 
the other products tested. Although the addition of this 
product to the SGF did not raise the pH significantly (Table 
I), approximately 35% of FK 506 was lost in 2 min and the 
FK 506 concentration remained constant for the remainder 
ofthe 24 h period (Fig. 4). This experiment was repeated with 
30 s sampling intervals over 5 min and a 35'Yo loss ofFK 506 
------------
IN-VITRO INTERACTION OF FK 506 AND ANTACIDS 577 
was noted within the first half min. Aluminium hydroxide 
powder. Amphogel (a brand of aluminium hydroxide gel) as 
well as other antacids containing aluminium hydroxide gel. 
namely Mylanta and Maalox. did not show a similar profile. 
This suggests that either an inactive ingredient in the 
suspension or the form of aluminium hydroxide gel used by 
Roxane may be adsorbing the FK 506 from the solution. 
In order to test this hypothesis, the solid pellet of 
aluminium hydroxide gel USP (Roxane) remaining after 
centrifugation of the 5 min sample was washed twice with 1 
mL ofSGF followed by a final wash with I mL acetonitrile. 
These washes were analysed for FK 506 as described above. 
The first wash contained approximately 15% of the original 
concentration of FK 506, the second wash 8%, and the last 
wash of acetonitrile contained 17% of FK 506. This 
represented a total recovery of 40% of the original amount of 
FK 506, approximately equal to the observed loss. This 
observation supports the hypothesis that FK 506 is adsorbed 
by aluminium hydroxide gel USP (Roxane) and that this 
process is reversible. 
Antacids can altcr drug absorption due to physical 
adsorption, changes in gastric pH. or their effect on gastric 
motility. Antacid-induced increase in the gastric pH may 
affect the disintegration. dissolution (captopril), solubility 
(iron). or ionization (salicylates) of enteric coated prepara-
tions and weakly acidic or basic drugs, and thereby alter their 
absorption (Gugler & AJlgayer 1990). Antacids can also alter 
gastrointestinal (GI) motility. Aluminium-containing ant-
acids and calcium carbonate slow GI motility and are 
constipating (American Society of Hospital Pharmacists 
1990; Gugler & Allgayer 1990). Aluminium salts may also 
delay gastric emptying. On the other hand, magnesium salts 
may increase GI motility (American Society of Hospital 
Pharmacists 1990). The potential effect of antacids on GI 
motility and therefore on FK 506 absorption could not be 
assessed in this in-vitro study. 
Antacids can also alter drug elimination by increasing the 
urinary pH, thereby enhancing or decreasing the tubular 
reabsorption of weakly basic or acidic drugs (American 
Society of Hospital Pharmacists 1990; Gugler & AUgaycr 
1990). Since FK 506 is completely eliminated by metabolism 
(Venkataramanan et al 1990) and is known to be a neutral 
compound (Kino et al 1987; Tanaka et al 1987) this is not a 
likely mechanism by which FK 506 and antacids may 
interact. 
The only other macrolide that is commercially available 
and commonly used is erythromycin. Antacids do not appear 
to alter the bioavailability of erythromycin significantly 
(Yamreudeewong et al 1989). Even though FK 506 and 
erythromycin are in the same class, their chemical structures 
and properties are quite different and one cannot predict in-
vivo FK 506 antacid interactions from this study. The results 
of this study suggest that. until in-vivo data are available, FK 
506 should be dosed separately from antacids in general and 
from Mag-Ox. This would avoid any potential interaction 
between these agents. 
References 
Abdallah, H. Y .. Venkataramanan, R .. Burckart. G. J .• Todo. S .• 
Starzl, T. E. (1990) The analysis and stability of FK 506 in 
aqueous media. Pharm. Res. 7: S3 
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p. 1620 
Fung, J. J., Todo, S .. Jain, A., McCauley. J., Alessiani. M .. Scotti. 
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M .. Takaya, S., Day, R., Gordon, R .. Starzl, T.E. et al (1991) 110 
consecutive primary orthotopic liver transplantations under FK 
506 in adults. Ibid. 23: 1397-1402 
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Nagase. K .. Krajack. A .. Imventarza. 0 .. TodD. S .. Fung. J. J .. 
Starz!. T. E. (1990) Pharmacokinetics of FK 506: preclinical and 
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United States Pharmacopeia (1980) USP XX. United States Phar-
macopeial Convention. Inc .. Rockville. MD 
Yamreudeewong. W., Scavone, J. M .. Paone. R. P .. Lewis. G. P. 
(1989) Effect ofanfacid coadministration on the bioavailability of 
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In-vitro Interaction of a Novel Immunosuppressant, FK 506, and Antacids

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In-vitro Interaction of a Novel Immunosuppressant, FK 506, and Antacids

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