New spectrophotometric methods for the determination of Minoxidil

 

Mukthinuthalapati Mathrusri Annapurna*, Polipalli Venkata Navya, Angirekula Narendra

GITAM School of Pharmacy, GITAM (Deemed to be) University, Visakhapatnam, India-530045.

 

ABSTRACT:

Minoxidil is a vasodilating agent used to treat pattern hair loss and resistant hypertension. New spectrophotometric techniques have been proposed for the estimation of Minoxidil in pharmaceutical dosage forms. Double beam Shimadzu Model UV-1800UV-VIS spectrophotometer was used for the present study. Minoxidil has shown linearity over the concentration range 0.1-20 µg/mL in distilled water, acetate buffer (pH 4.0), phosphate buffer (pH 5.8), borate buffer (pH 9.0), methanol and 0.1-15 µg/mL in phosphate buffer (pH 7.8) and the methods were validated as per ICH guidelines. The proposed methods are simple, economical and can be successfully applied for the assay of Minoxidil in pharmaceutical dosage forms.

 

 

INTRODUCTION: 

Minoxidil, an oral direct-acting peripheral vasodilator, lowers high systolic and diastolic blood pressure and is also applied topically to treat androgenetic alopecia¹. Minoxidil is a crystalline powder with molecular formula C₉H₁₅N₅O (Molecular weight 209.2483 g/mol). It is chemically 6-piperidin-1-ylpyrimidine-2, 4-diamine 3-oxide. It acts by causing vascular smooth muscle cells adenosine triphosphate-sensitive potassium channels to open and through the activation of extracellular signal-regulated kinase and the prevention of cell death. Minoxidil is believed to enhance the life of human follicle dermal papillary cells or hair cells. In the present study, the authors have proposed new spectrophotometric techniques for the assay of Minoxidil in topical solution and validated as per ICH guidelines.  Analytical methods such as spectrophotometry^2-3, liquid chromatography^4-7 and mass spectroscopy⁸ were developed for the determination of Minoxidil in pharmaceutical dosage forms and biological fluids. Similar type of spectrophotometric methods^9-13 have been developed for different drug molecules in the literature and they were all validated as per ICH guidelines.

 

Figure 1: Chemical structure of Minoxidil

 

MATERIALS AND METHODS

Shimadzu Model No. UV-1800 double beam UV-VIS spectrophotometer with quartz cells is used for the entire study, and all the solutions were scanned 200-400 nm. Buffer solutions such as acetate buffer (pH 4.0), phosphate buffer (pH 5.8, 7.8), borate buffer (pH 9.0) were prepared as per IP 2022. Minoxidil stock solution was prepared by dissolving 25mg of Minoxidil in 25mL volumetric flask with methanol (1000µg/mL), working standard solutions were prepared in methanol (100µg/mL) and further dilutions were prepared by diluting the working standard solutions with respective buffers as per the proposed methods requirement. Minoxidil is available as tablets and topical solution with brand names Lonitab Minoxidil Tablets (Intas Pharmaceuticals, Label claim: 2.5, 5 mg), Minoxiheart Tablets (STERIS Healthcare Pvt. Ltd. Label claim: 5 mg) and Mintop Forte (Dr. Reddy’s Laboratories, Label claim: 2, 5, 10% w/v), Minoqilib 5% Solution (Galderma India Pvt Ltd, Label claim: 5% w/v), respectively in India.

 

METHOD VALIDATION¹⁴

Zero order spectroscopy (D_o)

A series of Minoxidil solutions 0.1 - 20 µg/mL were prepared from working standard solution on dilution with distilled water (Method I), acetate buffer (pH 4.0) (Method II), phosphate buffer (pH 5.8) (Method III), borate buffer (pH 9.0) (Method V), methanol (Method VI) and 0.1 – 15 µg/mL for phosphate buffer (pH 7.8) (Method IV) and scanned (200-400 nm) against their reagent blanks. The zero order spectrum so obtained has shown maximum absorbance (λ_max) at 230 nm for Methods I, II, III, IV, V, and at 230.60 nm for Method VI. The absorbance of all the solutions was noted at λ_max, and a calibration curve was drawn by taking the concentration on the X-axis and the corresponding absorbance on the Y-axis for all the six methods respectively. Precision studies were performed by calculating the percentage relative standard deviation of independent assays of 6 determinations of the test concentration, whereas accuracy studies were carried out by standard addition method.

 

First order spectroscopy (D₁)

The individual zero-order absorption spectra of Minoxidil so obtained were converted into their first-order derivative spectra with the help of inbuilt software of the instrument for all the six methods. The resultant derivative spectrum has shown minima and maxima in all the six methods and therefore the amplitude were selected for the construction of calibration curves for Method I, II, III, IV, V, and VI.

 

Assay of Minoxidil Topical Solution

Minoxidil is available as topical solution (Intas Pharmaceuticals Ltd. India) (Dr. Reddy’s Laboratories Ltd. India) the topical solution was extracted with methanol, sonicated and diluted as per the requirement (10µg/mL) with distilled water (Method I), acetate buffer (pH 4.0) (Method II), phosphate buffer (pH 5.8) (Method III), phosphate buffer (pH 7.8) (Method IV), borate buffer (pH 9.0) (Method V), methanol (Method VI) and the and the percentage recovery was calculated as per the regression equations obtained.

 

RESULTS AND DISCUSSION:

Two different techniques – Zero-order (D₀) and First-order derivative spectroscopy (D₁) have been developed for the determination of Minoxidil in tablets and topical solution in six different reagents such as distilled water, acetate buffer (pH 4.0), phosphate buffer (pH 5.8, 7.8), borate buffer (pH 9.0) and methanol. The review of earlier established spectrophotometric methods were compared with the present method in Table 1.

 

Table 1: Comparison of previously reported UV spectroscopy methods with present methods

 

Zero order spectroscopy (D₀)

Minoxidil has shown absorption maxima at 230 nm for Method I, II, III, IV, V, and at 230.60 nm for Method VI. It obeys Beer-Lambert’s law over the concentration range 0.1-20 µg/mL (Table 2) for distilled water, acetate buffer (pH 4.0), phosphate buffer (pH 5.8), borate buffer (pH 9.0), methanol and 0.1 – 15 µg/mL for phosphate buffer (pH 7.8). The linear regression equations were found to be y = 0.1584x + 0.0032, y = 0.1251x + 0.0004, y = 0.1475x + 0.0076, y = 0.1664x - 0.0143, y = 0.1567x + 0.0017, y = 0.1734x + 0.0032 for distilled water, acetate buffer (pH 4.0), phosphate buffer (pH 5.8, 7.8), borate buffer (pH 9.0) and methanol, respectively. The absorption spectrum of Minoxidil (D_o) is given in Figure 2.

 

Table 2: Linearity of Minoxidil (Zero order spectroscopy)

*Mean of three replicates

 

Method I (Distilled water)	Method II (Acetate buffer pH 4.0)
Method III (Phosphate buffer pH 5.8)	Method IV (Phosphate buffer pH 7.8)
Method V (Borate buffer pH 9.0)	Method VI (Methanol)

Figure 2: Absorption spectra of Minoxidil (10 µg/mL) (D₀)

 

First order derivative spectroscopy (D₁)

The overlay first-order derivative spectra of Minoxidil in distilled water (Method I), acetate buffer (pH 4.0) (Method II), phosphate buffer (pH 5.8) (Method III), phosphate buffer (pH 7.8) (Method IV), borate buffer (pH 9.0) (Method V), methanol (Method VI) were shown in Figure 3 and the spectral characteristics observed were shown in Table 3. The derivative spectrum has shown both maxima and minima and therefore the amplitude was taken against concentration and calibration curves were drawn. Minoxidil obeys Beer-Lambert’s law over the concentration range 0.1-20 µg/mL for distilled water, acetate buffer (pH 4.0), phosphate buffer (pH 5.8), borate buffer (pH 9.0), methanol and 0.1 – 15 µg/mL for phosphate buffer (pH 7.8) respectively and the linear regression equations that are found to be y = 0.0114x + 0.0008, y = 0.0105x + 0.0003, y = 0.011x + 0.0003, y = 0.0128x - 0.0008, y = 0.0121x - 0.0001, y = 0.0143x + 0.0017, respectively.

The percentage RSD in accuracy (Table 4) and precision studies (Table 5-6) for all the methods was found to be less than 2 in indicating that the methods are precise and accurate. The assay was performed for the Minoxidil topical solution and the percentage recovery was calculated (Table 7). The optical characteristics of the method were shown in Table 8.

 

 

Method I (Distilled water)	Method II (Acetate buffer pH 4.0)
Method III (Phosphate buffer pH 5.8)	Method IV (Phosphate buffer pH 7.8)
Method V (Borate buffer pH 9.0)	Method VI (Methanol)

Figure 3: Absorption spectra of Minoxidil (10 µg/mL) (D₁)

 

Table 3: Linearity of Minoxidil (First derivative spectroscopy) (Max: Maxima; Min: Minima)

 

Table 4: Accuracy of Minoxidil Topical solution

*Mean of three replicates

 

 

Table 5: Precision study of Minoxidil Topical solution (Zero order spectroscopy)

*Mean of three replicates

 

Table 6: Precision study of Minoxidil Topical solution (First derivative spectroscopy)

*Mean of three replicates

 

 

Table 7: Assay of Minoxidil Topical Solution

*Mean of three replicates

 

Table 8: Optical characteristics of Minoxidil – Zero order spectroscopy

 

CONCLUSION:

All the spectrophotometric techniques were validated and found to be very simple, accurate, precise, and economical. These methods can be conveniently used for the routine analysis of Minoxidil Pharmaceutical formulations.

 

ACKNOWLEDGEMENT:

The authors are grateful to Intas Pharmaceuticals Ltd. for providing the gift samples of Minoxidil and M/s GITAM (Deemed to be) University for providing the research facilities.

 

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Accepted on 28.04.2024           © RJPT All right reserved

Research J. Pharm. and Tech 2024; 17(5):2372-2378.