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Three Methods for theQuantitative Determination ofMinoxidilMohamed El-Sayed Mahrous aa Pharmaceutical Chemistry Department, Facultyof Pharmacy, University of Alexandria, Alexandria,EgyptPublished online: 23 Oct 2006.

To cite this article: Mohamed El-Sayed Mahrous (1991): Three Methods for theQuantitative Determination of Minoxidil, Analytical Letters, 24:11, 2017-2032

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ANALYTICAL LETTERS, 2 4 ( 1 1 ) , 2017-2032 (1991)

TZIREE METB)DS H)R THE WANTITATXVE

DETERMINATION OF MINOXIDIL

Key Words: Uinox id i l , Pharmaceut ical Formulation

Spectrophotometry, Fe r ro in Complex,

I o d i n e , Chlorani l .

Mohamed El-Sayed Hahrous,

Pharmaceut ical Chemistry Department, F a c u l t y o f

Pharmacy, Un ive r s i ty of Alexandria , Alexandria , Egypt.

Abs t r ac t :

Three new methods a r e desc r ibed f o r t h e q u a n t i t a t i v e

de t e rmina t ion o f minoxidi l . The first method i s based on

t h e formation of T r i s (0 -phenan th ro l ine ) i r o n (11) complex

( f e r r o i n ) upon r e a c t i o n of minox id i l with an i r o n (111)

-0-phenanthrol ine mix tu re i n s l i g h t l y a c i d i c medium. The

f e r r o i n complex i s c o l o r i m e t r i c a l l y measured a t 510 nm.

The second method is based o n . t h e i n t e r a c t i o n of

minox id i l as an e l e c t r o n donor with i o d i n e as e l e o t r o n

a c c e p t o r t o g i v e a h i g h l y co lou red molecu la r cha rge

2017

Copyright 0 1991 by Marcel I k k k e r , Inc

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t r a n s f e r complex which e x h i b i t s maximum a b s o r p t i o n a t

285 nm.

The t h i r d method i s based on t h e i n t e r a c t i o n o f

minox id i l and c h l o r a n i l i n dioxan l e a d i n g t o t h e forma-

t i o n of q u a t e r n i z e d complex e x h i b i t i n g b l u i s h g reen

co lour wi th maximum a b s o r p t i o n a t 610 nm.

The op t ima l expe r imen ta l p a r a m e t e r s f o r t h e c o l o u r

p roduc t ion o f each method are s e l e c t e d . Beer ’ s law i s v a l i d w i t h i n a c o n c e n t r a t i o n r ange

of 0.08-0,4ag%, 0.1 - 0.5 mgs and 5 - 25 mg% r e s p e c t i v e l y .

The developed methods are s imple, s e n s i t i v e , a c c u r a t e

and s u i t a b l e f o r t h e a s s a y o f m i n o x i d i l i n bulk powder

and i n pharmac eu t i c a l fo rmula t ions .

I n t r o d u c t i o n :

Minox id i l (2,l+-diamino-6-piperidinopyrimidine 3-

ox ide ) i s a p o t e n t a n t i h y p e r t e n s i o n drug. I t i s i n d i c a t e d

f o r o r a l t r e a t m e n t of s e v e r e symptomatic o r organ - damag-

i n g h y p e r t e n s i o n t h a t i s n o t c o n t r o l l e d with any o t h e r

drug o r combination o f drugs. I t a c t s as a d i r e c t - a c t i n g p e r i p h e r a l v a s o d i l a t o r t h a t d e c r e a s e s both s y s t o l i c

and d i a s t o l i c blood p r e s s u r e by d e c r e a s i n g p e r i p h e r a l

v a s c u l a r r e s i s t an c e ’ -3.

I t i s also e f f e c t i v e t o p i c a l l y a s a h a i r growth

s t i m u l a n t and i s i n d i c a t e d f o r t h e t r e a t m e n t of a l c o p e c i a

androgent icab.

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DETERMINATION OF MINOXIDIL 2019

Few methods were r e p o r t e d f o r t h e q u a n t i t a t i v e

d e t e r m i n a t i o n of minox id i l . I t was determined c o l o r i -

m e t r i c a l l y u s i n g copper n i t r a t e 5 , and spec t ropho to -

m e t r i c a l l y 6 a t 231 o r 287 nm.

h igh performance l i q u i d chromatography 9-14 and t h i n

l a y e r chromatography’

t i v e d e t e r m i n a t i o n e i t h e r i n t h e pha rmaceu t i ca l formula - t i o n s o r i n t h e b i o l o g i c a l f l u i d s . Minox id i l was a l s o

determined by polarography’

Gas ~ h r o m a t o g r a p h y ~ ’ ~ ,

were a l s o used f o r i t s q u a n t i t a -

and by radioumminoassay 17 . T h i s work h a s undertaken t h e development of new

f a c i l e and r a p i d s p e c t r o p h o t o m e t r i c p rocedures f o r t h e

d e t e r m i n a t i o n o f microgram amounts o f m i n o x i d i l a6

p r e s e n t i n t h e pu re form o r i n pha rmaceu t i ca l fo rmula t ions .

Experimental

Apparatus:

A Perk in - Elmer, Model 550 S spec t ropho tomete r

w i t h matched 1-cm q u a r t z c e l l s .

M a t e r i a l and r e a g e n t s :

1-

2-

3-

Pure m i n o x i d i l bulk powder, ( Up john Co. ) . P r e p a r a t i o n o f i r o n (111) -0 -phenan th ro l ine r e a g e n t :

mix 0.198 g of t h e o -phenan th ro l ine monohydrate

2 m l o f 1 N HC1 and 0.16 g o f f e r r i c ammonium s u l p h a t e

and d i l u t e with water t o 130 m l .

I o d i n e s o l u t i o n ( 5 x 10-3M): Prepa red by d i s s o l -

ving 1 9 mg i o d i n e i n chloroform o f spec t ropho to -

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m e t r i c grade, i n 100 ml vo lumet r i c f l a s k . The

s o l u t i o n i s s t a b l e f o r 4 weeks when k e p t a t 4 C,

p r o t e c t e d from s u n l i g h t .

0

4- A 0.5% w/v s o l u t i o n o f c h l o r a n i l i n dioxan.

5- Sodium a c e t a t e - a c e t i c a c i d b u f f e r t 8 (pE = 5 ) :

mix 14.8 m l o f 0.2 M a c e t i c a c i d (12.0 g/L) w i t h

35.2 ml o f 0.2 M sodium a c e t a t e (16.4 g C,H702Na/L)

and d i l u t e with d i s t i l l e d wa te r t o 100 ml.

Procedures:

1- I r o n (111)-o-phenanthrol ine method ( f i r s t method)

M s s o l v e 20 mg minox id i l powder i n 50 m l

methanol i n t o 50 ml vo lumet r i c f l a s k ( 1 m l = 0.4 mg).

T r a n s f e r 5 ml from t h i s s t o c k s o l u t i o n i n t o 100 ml

volumetr ic f l a s k and complete t o 100 m l w i th t h e

b u f f e r ( 1 m l = 0.02 mg). Aliquot volumes 1-5 ml

of t h i s s o l u t i o n ( c o n t a i n i n g 0.02 - 0.1 mg m i n o x i d i l )

were t r a n s f e r r e d t o 25 m l vo lumet r i c f l a s k s , d i l u t e d

t o abou t 5 ml with b u f f e r and then t r e a t e d with 2 ml

reagent . The f l a s k s were hea ted i n a b o i l i n g water

ba th f o r 15 minutes , cooled, then completed t o t h e

mark with water. “he r ed c o l o u r developed was

measured a t 510 nm, a g a i n s c a r e a g e n t blank t r e a t e d

s i m i l a r l y ( M g 1). T h i s p rocedure was a p p l i e d f o r

t h e d e t e r m i n a t i o n o f minox id i l i n bu lk powder and i n

i t s t a b l e t s .

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DETERMINATION OF MINOXIDIL 2021

A

0 . 5 .

0.4

0.3

0. i

0.1

Fig 1: Absorption spectra Of : A ) 20 mg% minoxidil with chlorrnil. B) 0.2 mg% minoridi l with Fa (1111-0-

phcnanthroline.

2- Iodine method (second method)

Dissolve 10 mg minoxidil powder accurately

weighed i n -100 m l aethanol in to a 100 m l volumetric

f lask ( 1 m l = 0.1 me). Transfer 0.1 - 0.5 m l

(containing 0.01 - 0.05 mg minolddil) i n t o 10 61

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V O l U D e t r i C f l a s k s and dilute to volume w i t h abou t

0.5 m l w i t h methanol, t hen t r e a t e d w i t h 2 ml r e a g e n t .

Allow t o s t a n d f o r 15 minu tes then complete t o t h e

mark with chloroform. Measure t h e abso rbance a t 285

nm, a g a i n s t a r e a g e n t blank t r e a t e d s i m i l a r l y (Fig 2 ) .

T h i s p rocedure was a p p l i e d f o r t h e d e t e r m i n a t i o n of

minox id i l i n bluk powder and i n i t s t a b l e t s .

3- C h l o r a n i l method (Th i rd method)

Al iquo t volumes of 0.5-2.5 m l o f an a l c o h o l i c

s o l u t i o n o f minox id i l ( 1 ml = 0.5 mg) were t r a n s -

f e r r e d t o 10 m l vo lumet r i c flasks and d i l u t e d t o

abou t 2.5 r n l wi th a l c o h o l , t hen completed t o 5 m l

wi th dioxan. Two m l of c h l o r a n i l r e a g e n t was a d d e d

t o each f l a s k . The f l a s k s were h e a t e d on a b o i l i n g

water bath f o r 5 minutes , cooled, and t h e volume

completed t o t h e mark w i t h dioxan. The develoned

b l u i s h g reen c o l o u r was measured a t 610 nm, a g a i n s t

a r e a g e n t b l ank t r e a t e d s i m i l a r l y ( F i g 1 1. T h i s

procedure was a p p l i e d f o r t h e d e t e r m i n a t i o n o f

minox id i l i n bu lk powder and i n i t s t a b l e t s .

Results and M s c u s a i o n :

The f i r s t method i s based on t h e fo rma t ion o f T r l s

(0 -phenan th ro l ine ) i r o n ( 11) c h e l a t e upon r e a c t i o n o f

m i n o x i d i l with an i r o n (111) -0-phenanthrol ine mixture .

The r e a c t i o n p r o c e e d s through r e d u c t i o n of i r o n (111)

i o n s t o i r o n (11) and subsequent fo rma t ion of t h e i n t -

e n s i v e o range - red c o l o u r a t i o n of t h e f e r r o i n complex.

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DETERMINATION OF MINOXIDIL 2023

A 0 .6

0.5

0 . 4

0.3

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0.1

Fig 2: Abaorption spectrum of 0.3 mq% minoxidil with iodine.

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F e r r o i n Complex

F a c t o r s a f f e c t i n g t h e r e a c t i o n , namely nH, h e a t i n g

t ime and r e a g e n t c o n c e n t r a t i o n , were c a r e f u l l y s t u d i e d

t o ach ieve q u a n t i t a t i v e r e s u l t s . Sodium a c e t a t e - a c e t i c

a c i d b u f f e r s o l u t i o n s ?/ere t r i e d and t h e maximum c o l o u r

i n t e n s i t i e s were o b t a i n e d a t pE v a l u e s between 4.5 and

6.0. The s t u d y o f t h e h e a t i n g time r e v e a l e d t h a t 15

minu tes were enough f o r t h e complete c o l o u r development

a t pH 5 ( F i g 3). Also 2 m l of r e a g e n t c o n c e n t r a t i o n

w a s s u f f i c i e n t f o r complete c o l o u r fo rma t ion ( F i g 4) .

The c o l o u r formed under t h e s e c o n d i t i o n s was found

t o be s t a b l e f o r more than 24 hours .

Beer’s l a w is v a l i d w i t h i n t h e c o n c e n t r a t i o n r ange

o f 0.08 - 0.4 mg% o f t h e f i n a l d i l u t i o n and t h e molar

a b s o r p t i v i t y i s 4.4 x lo4 L .mole-’. cm”.

The second method depends upon t h e i n t e r a c t i o n

between t h e m i n o x i d i l and i o d i n e r e s u l t i n g i n t h e forma-

t i o n o f an i n t e r m o l e c u l a r cha rge t r a n s f e r complex i n v o l -

v i n g an e l e c t r o n t r a n s f e r from donor ( m i n o x i d i l base) t o

accep to r19 ( i o d i n e ) . Minox id i l - i o d i n e complex gave

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DETERMINATION OF MINOXIDIL 2025

A

0.8

0.6

0-4

0.2

, , /

.

5 10 15 20 mlnu t e 6

Fig. 3. Effect of time on the colour development using: A) 0.4 mg% minoxidil with iron(II1)-0-phenanthroline and heating

on boiling water bath. B ) 0.3 mg% minoxidil with iodine on cold. C) 20 mg% minoxidil with chloranil and heating on boiling water

bath.

s t r o n g a b s o r p t i o n maximum a t 285 nm ( F i g 2). The r e a c t i o n

m i x t u r e shou ld s t a n d f o r 30 m i n u t e s p r i o r t o abso rbance

measurement, t o minimize change i n t h e abso rbance w i t h

t ime due t o t h e conve r s ion o f o u t e r cha rge - t r a n s f e r com-

p l e x of i o d i n e . A f t e r t h a t p e r i o d t h e complex i s s t a b l e

f o r 3 minu tes ( F i g 3). The c o n c e n t r a t i o n o f i o d i n e i n

20

ch lo ro fo rm was deduced t o be 2 m l o f ( 5 x lO'jM) whereas

f u r t h e r i n c r e a s e i n c o n c e n t r a t i o n had no e f f e c t on t h e

i n t e n s i t y o f a b s o r p t i o n r e a d i n g s ( F i g 4 ) .

B e e r ' s law i s v a l i d w i t h i n t h e c o n c e n t r a t i o n r ange

o f 0.1 - 0.5 mg% o f t h e f i n a l d i l u t i o n and t h e molar

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EL-SAYED MAHROUS

,, : ,'

/ ' / ,

F i g . 4 . E f f e c t of volume of r e a g e n t s on t h e c o l o u r development u s i n g :

A) 0.4 mg% m i n o x i d i l w i t h Fe(II1)-0-phenanthroline. B) 0 .3 mg% m i n o x i d i l w i t h i o d i n e . C ) 20 mg% m i n o x i d i l w i t h c h l o r a n i l .

a b s o r p t i v i t y i s 4.5 x 10 4 L.mole-'. cm". S i n c e m i n o x i d i l

is s l i g h t l y s o l u b l e i n chloroform, and t h e p r e s e n c e o f

h i g h amount of methanol w i l l i m p a r t a ye l low c o l o u r t o

t h e r e a c t i o n mix tu re , t h e r e f o r e t h e volume of methanol

i n t h e r e a c t i o n i s r e s t r i c t e d t o 0.5 m l .

I n t h e t h i r d method, t h e p rocedure r e p o r t e d f o r t h e 21,22 a s s a y of p r imary and secondary amine u s i n g c h l o r a n i l

was t r i e d f o r t h e d e t e r m i n a t i o n o f m i n o x i d i l b u t w i thou t

any S U C C ~ ~ S . The p rocedure of S a s s e t a123 f o r determi-

n a t i o n o f s imple t e r t i a r y amines by warming wi th c h o r a n i l

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DETERMINATION OF MINOXIDIL 2021

i n t o l u e n e a s a s o l v e n t a l s o f a i l e d t o g ive any co lou r .

Dioxan as a s o l v e n t and h e a t i n g were t r i e d and under t h e s e

c o n d i t i o n s c h l o r a n i l gave a c o l o u r w i t h m i n o x i d i l measured

a t 610 nm (Fig 1 ) .

S a s s e t a123 and R - S e b a i e t a124 proved t h e react-

t i o n between c h l o r a n i l and t e r t i a r y amines t o be q u a t e r -

n i z a t i o n r e a c t i o n with t h e l i b e r a t i o n o f c h l o r i d e i o n :

Chlo r a n i 1 T e r t i a r y amine Complex

F a c t o r s a f f e c t i n g t h e c o l o u r fo rma t ion such a s

r e a g e n t c o n c e n t r a t i o n & time of h e a t i n g were s t u d i e d

c a r f u l l y i n o r d e r t o maximize t h e c o n d i t i o n o f t h e

r e a c t i o n . A 2 m l of 0.5% w/v c h l o r a n i l s o l u t i o n i n

dioxan was found enough f o r complete c o l o u r develep-

ment, whereas f u r t h e r i n c r e a s e i n c o n c e n t r a t i o n had

no e f f e c t on t h e i n t e n s i t y of abso rbance (Fig 4). The

h e a t i n g time was found t o be 4 m i n u t e s and any f u r t h e r

i n c r e a s e i n t ime gave c o n s t a n t a b s o r p t i o n r e a d i n g s ( N g 3 ) . Beer ‘ s l a w i s v a l i d i n t h e c o n c e n t r a t i o n r ange of

The molar a b s o r p t i v i t y i s 6 x 10 2 0.5 - 2.5 mg/ 10 ml.

L.rnole-' . cm” .

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DETERMINATION OF MINOXIDIL 2029

The d e r i v a t i o n o f r e g r e s s i o n e q u a t i o n s o f t h e c a l i b -

r a t i o n curves were:

A = 0.002 + 2.14 C , r = 0.9999 f o r t h e f i r s t method

A =-O.OOl+ + 2.188 C , r = 0.9998 f o r t h e second method

A = 0.005 + 0.02482 C and r=O.gggg f o r t h e t h i r d method

Where C is i n mg$. These e q u a t i o n s were u t i l i z e d f o r

computat ion o f d i f f e r e n t unknown d rug c o n c e n t r a t i o n .

The developed methods gave concordan t r e s u l t s x i t h

good r e p r o d u c i b i l i t i e s .

The mean % r e c o v e r i e s showed no d i f f e r e n c e when com-

p a r i n g t h e t h r e e methods with each o t h e r , e n s u r i n g t h e

a c c u r a c y of t h e adopted methods ( T a b l e 1 ) . The proposed

methods a r e n o t i n t e r f e r e d by t a b l e t f i l l e r s fo rmula t ed

wi th minox id i l .

A s shown from t h e t a b l e , t h e s e n s i t i v i t y of t h e

p rocedures a r e i n t h e f o l l o w i n g o r d e r , f i r s t method > second method > t h i r d method.

A f u r t h e r check on t h e v a l i d i t y o f t h e proposed

methods was done by a n a l y z i n g monoxidi l bulk powder and

i t s t a b l e t s u s i n g UV s p e c t r o p h o t o m e t r i c method . S t a t i s t i c a l a n a l y s i s of t h e r e s u l t s r e v e a l e d t h a t t h e r e

i s no s i g n i f i c a n t d i f f e r e n c e between them ( T a b l e 1) .

2 5

The sugges t ed methods have t h e advantage of b e i n g

s imple, a c c u r a t e and s e n s i t i v e and may be c o n s i d e r e d as

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20 30 EL-SAYED MAHROUS

g e n e r a l methods f o r s p e c t r o p h o t o m e t r i c d e t e r m i n a t i o n

of minox id i l .

Re fe rences

1 - V.M. Campese, Drugs, 22, 257 (1981).

2- Minoxidi l ( L o n i t e n ) , t h e Medical L e t t e r , - 22, 21 (1980).

3- D.T. Loaen tha l and M.B. Affr ime, J. Cardiavasc.

Pharmacol, z, S93 (1980).

4- J.A. Rumsfield, D.P. West and V.C. F i e d l e r - Webs,

C l i n i c a l Pharmacy, 6, 386 (1987) . 5- A. Araman, Acta Pharmaceut ica T u r c i c a , 2, 9 (1986).

6- The Uni t ed S t a t e s Pharmacopeia, G s c r e v i s i o n - The

N a t i o n a l Formulary, 5 t h Ed., Mack p u b l i s h i n g

company, Eascon, PA, U.S.A., supplement 3, 2193 (1986).

7- T. Kanierwska, E. Kubl in and E. Weclawowiez, Acta

Pol. Pharm., 43, 588 (1986). - 8- T. Kanieruska, E. Kublin and B.Ewiatkowska, Farm. Pol ,

- k 3 , 508 (1987).

9- P.A. Asmus, J.B. Land i s , M.E. G r a n t and H.A. Havel,

J. Pharm S c i , 73, 1290 (1984) - 10- G. Carrum, D.R. Abernethy,and M. Sadhukhan, J.

Chromatogr., 381, 127 (1986). - 1 1 - G.A. Johnson, K. J. Barsuhn and J . M . Mc C a l l , Drug

Metab. Dispos., 2, 507 (1983).

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DETERMINATION OF MINOXIDIL 2031

12- G . A . Johnson , K . J . Barsuhn and J . M . Mc Call , Biochem.

Pharmacol. , 2, 2949 ( 1982).

13- G.A. Johnson and C . A . Baker, C l i n i c a Chimica Acta.,

- 169, 217 (1987).

14- M.H. Golden and P.H. Zoutendam, J. Pharm. BLomed.

Anal.,5, 543 (1987).

15- A.H. S t e a d , R. G i l l , T. Wr igh t , J.P. G ibbs and A.C.

Moffa t , A n a l y s t , 107, 1106 (1982).

16- L. Amankwa, L.G. C h a t t e n and S. Pons, A n a l y s t 108,

1221 (1983).

17- M.E. Royer, H. HO, T.J. G i l b e r t s o n , J .M. Maca l l ,

K.T. J o h n s t o n and R. S t r y d , J. Pharm. Sc i . , 66, 1267 (1977).

18- G.S. Walpole, J. Chem. SOC., 105, 2501 (1914).

13- R. F o s t e r "Organic c h a r g e t r a n s f e r complexes"

-

Academic P r e s s , London, 23, 33 ( 1 9 6 9 ) . - XI- C.N.R. Rao, S.N. Bhat and P.O. Dwivedi "ApDlied

S p e c t r o s c o p y Reviews", volume 5, E.G. Brame, Ed.,

Marcs l Dekker, N e w York (1972) .

31- F. Al-Sulimany and A . Townshend, Anal. C h i m Acta ,

- 4 6 , 195 (1973).

22- T.S. .a-Ghabsha and S.A. Rahim, i b i d . , 2_5, 189 (1976).

23- S. Sass, J.J. Kaifman, A . A . Cardenas and J.J. M a r t i n ,

Anal. Chem., 30, - 529, (1958).

24- El -Sebai A. I b rah im, A.S. Issa, M.A. Abdel Salam

and M.S. Mahrous, T a l a n t a , 30, 531 (1983).

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20 32 EL-SAYED MAHROUS

25- A.C. Moffat, Ed., C l a r k e ' s I s o l a t i o n and I d e n t i f i c a t i o n

o f Drugs, The Pharmaceut ica l P r e s s , London, pp. 786-787.

Received February 1 1 , 1991 Accepted J u l y 1 , 1991

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