Minoxidil: A Comprehensive Review of its Mechanism, Efficacy, Safety in Treating Hair Disorders

Minoxidil was discovered in 1970 and originally developed to treat hypertension, but its unexpected side effect of hair regrowth led to its use in treating androgenetic alopecia. This discovery prompted the creation of a topical formulation, which was first introduced in a 2% concentration and later in a stronger 5% formula. ⁽¹⁾ Millions of people in the US experience pattern hair loss, with an estimated 50 million men and 30 million women affected. Genetic predisposition and androgen receptor variations play a significant role in this condition. Currently, two FDA-approved treatments exist: Oral Finasteride, a 5-alpha-reductase inhibitor that reduces DHT levels and is effective for men, and Topical Minoxidil, a solution that stimulates hair growth. ⁽⁷⁾ Initially used as an antihypertensive, minoxidil's unexpected side effect of hypertrichosis led to its repurposing as a hair growth agent. ⁽⁶⁾ Minoxidil promotes hair regrowth through increased cutaneous blood flow and potassium channel activation. While its mechanism of action is not fully understood, its efficacy has been consistently demonstrated. Common adverse effects include facial hypertrichosis and contact dermatitis, scalp pruritus, scalp scaling. Beyond its primary use for AGA, minoxidil has been used off-label for other types of alopecia, including Telogen effluvium (TE), Alopecia areata (AA), Frontal fibrosing alopecia (FFA), Central centrifugal cicatricial alopecia (CCCA), Traction alopecia (TA), Eyebrow hypotrichosis, Monilethrix, Chemotherapy-induced alopecia (CIA). ⁽²⁾ For a long time, oral minoxidil wasn't typically used for hair loss due to concerns about its side effects at higher doses. One of the main issues was the tendency to cause sodium and fluid retention, which could lead to oedema and weight gain. To mitigate this, doctors often prescribed beta blockers to help manage these effects and control heart rate. While oral minoxidil has been linked to some serious cardiac conditions, more recent research has focused on using low-dose, which seem to reduce these risks while still providing benefits for hair loss treatment. This shift has sparked renewed interest in oral minoxidil's potential for treating alopecia. ⁽³⁾ Androgenetic alopecia or male-pattern hair loss occurs when hair follicles shrink due to the influence of dihydrotestosterone, leading to progressive hair loss. This condition affects many people, with its prevalence increasing with age - approximately 73% of men and 57% of women over 80 are affected. Research suggests that this condition can have emotional impacts such as feelings of self-conscious preoccupation and diminished attractiveness. Currently, there are FDA-approved medications like minoxidil and finasteride, as well as low-level laser light therapy (LLLLT), available to address this condition. However, a comprehensive analysis of the effectiveness of these treatments has not been conducted. ⁽⁵⁾

Pharmacological insights of Minoxidil:

Figure 1: Chemical structure of Minoxidil

Minoxidil's chemical structure (2,6-diamino-4-piperidinopyrimidine-1-oxide) enables it to effectively promote hair growth. The Minoxidil Solution (MS) formulation contains ingredients like ethanol and propylene glycol (PG) to enhance solubility, but these can cause irritation. A newer PG-free Minoxidil Foam (MF) formulation was developed to minimize irritation while improving delivery. This foam version is more convenient, dries quickly, and targets the affected area more precisely. Minoxidil works by widening blood vessels and stimulating cell growth through potassium channel activation. Its active metabolite, minoxidil sulfate, produced by the enzyme sulfotransferase in hair follicles, is key to its effectiveness. However, individual variations in enzyme activity can impact treatment response. Certain medications, like aspirin, can reduce the enzyme's activity, diminishing minoxidil's benefits. When applied topically, minoxidil is minimally absorbed systemically, and most of the absorbed amount is eliminated through the kidneys. ⁽¹⁾

Physiological actions of Minoxidil:

Long-term use of Minoxidil has shown to effectively promote hair growth and reduce hair loss. When the treatment is discontinued, hair loss tends to resume within 12 to 24 weeks. Studies have found that Minoxidil accelerates the transition of hair follicles from the telogen phase to the anagen phase, leading to increased hair density. Clinical trials have demonstrated significant improvements in hair growth and reduced hair loss, particularly at higher concentrations (5% formulation). The medication appears to work by enhancing blood flow and stimulating cell growth in the hair follicles through β-catenin activity and increased VEGF expression. It also increases the production of factors that promote perifollicular vascularization and hair growth. Additionally, it influences the production of prostaglandin E2, which helps maintain the anagen phase. Overall, Minoxidil has been shown to be effective in promoting hair growth and reducing hair loss. ⁽¹⁾

Effect of Minoxidil oh Hair Follicle Dynamics:

The process by which minoxidil fosters hair growth is multifaceted, potentially encompassing several key actions, including prolonging the anagen phase (active growth phase) of the hair follicle cycle, shortening the telogen phase (resting phase), and increasing hair diameter by enhancing the thickness of individual hair strands. Through these actions on the hair cycle and hair fibre thickness, minoxidil promotes hair growth, enhances hair density, and increases the length and diameter of hair fibres. Although the precise molecular details of its mechanism remain unclear, its impact on the hair cycle and fibre diameter plays a significant role in its effectiveness for addressing hair loss conditions such as androgenetic alopecia (AGA). ⁽⁹⁾

Impact of Minoxidil in Humans:

The impact of minoxidil on typical human hair growth remains somewhat enigmatic, with the majority of studies examining its efficacy in addressing androgenetic alopecia (AGA). In cases of male pattern baldness (male AGA), minoxidil has demonstrated a swift increase in hair growth, noticeable within 6-8 weeks of initiating treatment and reaching its peak effect around 12-16 weeks. This prompt response is likely attributed to minoxidil's ability to stimulate follicles in the telogen phase (resting phase) to transition into the anagen phase (active growth phase), rather than reversing the miniaturization of hair follicles. Additionally, minoxidil may extend the duration of the anagen phase, as suggested by the increased hair length observed in areas affected by hypertrichosis (excessive hair growth). Histopathological investigations have yielded mixed findings, with some studies indicating an increase in the anagen/telogen ratio and an augmentation in mean hair diameter, while others have reported less definitive results. The observed increase in mean hair diameter might be the result of preferential recruitment of large diameter hairs rather than individual follicle hypertrophy. Given the complexity of minoxidil's mechanism of action, further research is necessary to gain a comprehensive understanding of its effects on human hair growth and its potential applications in treating hair loss conditions. ⁽⁹⁾

Impact of Minoxidil in Animals:

Studies have explored the impact of minoxidil on hair growth using different experimental models. In one investigation involving rats, topical application of minoxidil was found to reduce the duration of the telogen phase (resting phase), enabling hair follicles to transition into the anagen phase (active growth phase) more rapidly, while not influencing the length of anagen itself. In a primate model, the stump-tailed macaque, which exhibits androgenetic alopecia (AGA) closely resembling human balding, topical minoxidil was shown to prevent scalp hair loss and stimulate regrowth in animals experiencing baldness. Histological examination demonstrated that minoxidil treatment resulted in an increased proportion of hair follicles in anagen, a decrease in telogen follicles, and an enlargement of hair follicle size. These results imply that minoxidil's mechanism of action involves regulating the hair growth cycle and promoting hair growth, making it a potential therapeutic option for treating AGA and other hair loss conditions. ⁽⁹⁾

Minoxidil Sulphation:

The sulphated derivative of minoxidil, minoxidil sulphate, is the active form responsible for the drug's antihypertensive effects, achieved through the relaxation of vascular smooth muscle. This biochemical reaction is facilitated by sulphotransferase enzymes. Research has identified minoxidil sulphotransferase activity in various tissues, including rat liver, human liver, platelets, and keratinocytes, as well as in mouse vibrissae follicles, rat pelage, and rat keratinocytes. In the scalp skin of stump tail macaques, sulphotransferase activity is largely localized in the hair follicle. Human cytosolic sulphotransferase enzymes are encoded by five known genes, responsible for sulphating phenols and catecholamines, oestrogens, and hydroxysteroids. Notably, interindividual variations in scalp sulphotransferase activity are correlated with activity levels in platelets. The mechanism of action of minoxidil sulphate involves the opening of adenosine triphosphate (ATP)-sensitive potassium channels (KATP channels) in the plasmatic membrane. These KATP channels are present in multiple tissues and cell types, including heart, pancreas, vascular smooth muscle, and central nervous system. Potassium channel activity may also be required for early-stage cell proliferation by G1 progression of the cell cycle. Minoxidil may increase DNA synthesis, as demonstrated in hepatocyte cultures. Other potassium channel openers, such as diazoxide and pinacidil, can cause hypertrichosis in humans, similar to minoxidil. Studies have shown that minoxidil, Cromalin, and P-1075 (a pinacidil analogue) stimulate hair growth in balding macaques, and increase thymidine uptake in hair growth cultures of mouse vibrissae follicles and human hair follicle organ cultures. ⁽¹⁰⁾

Mechanism of Action of Minoxidil:

Minoxidil, appears to have a multifaceted impact on hair growth, influencing various biological processes. Its effects may include vasodilation to improve blood flow, anti-inflammatory properties to reduce inflammation, and modulating the Wnt/β-catenin signalling pathway that regulates hair follicle activity. Additionally, it may impact the duration of the anagen (hair growth) and telogen (resting) phases, contributing to its overall efficacy in promoting hair growth. By targeting multiple mechanisms, this treatment may achieve a more comprehensive effect on hair follicle health, also exhibiting antiandrogen effects. ⁽⁶⁾

1.Circulatory Enhancer:

The action of Minoxidil triggers a response that opens ATP-sensitive potassium (KATP) channels, leading to vasodilation and improving circulation. This enhanced blood flow may provide hair follicles with increased access to vital nutrients and oxygen. As a result, hair growth may be stimulated, and follicle health improved. By easing vascular tension and reducing systolic and diastolic blood pressure, this treatment may also have a beneficial effect on blood pressure, which could further support its role in promoting hair growth. ⁽⁶⁾

2.Inflammatory Suppressant:

One theory is that Minoxidil fosters hair growth by mitigating perifollicular microinflammation. Studies have shown that it can suppress T-lymphocyte activity and decrease the production of pro-inflammatory substances, such as interleukin-1α and prostacyclin. By reducing localized inflammation, this treatment may promote a healthier environment for hair to grow. This potential anti-inflammatory mechanism could be a key factor in its effectiveness for addressing hair loss. Furthermore, reduced inflammation may lead to increased hair density and growth rate, and anti-inflammatory effects may help counteract the negative impact of inflammation on hair follicle stem cells. Additionally, Minoxidil's anti-inflammatory properties may complement its vasodilatory effects, enhancing overall hair growth promotion, which suggests that its anti-inflammatory action plays a significant role in its therapeutic benefits for hair loss treatment. ⁽⁶⁾

3.Activator of wnt/b-catenin signalling

Minoxidil may promote hair growth by triggering the production of Vascular Endothelial Growth Factors (VEGF) that stimulate vascularization. VEGF can activate the β-catenin signalling pathway, which supports the development and regeneration of hair follicles through the Wingless-Int (Wnt) pathway. Additionally, the increased vascularization may help prolong the anagen phase of hair follicles, particularly in cases of Androgenetic Alopecia (AGA), leading to improved hair growth and regeneration. This mechanism is further supported by the potential benefits of enhanced vascularization, including increased oxygen and nutrient delivery to hair follicles, VEGF's role in angiogenesis supporting healthy hair follicle growth and maintenance, and the crucial involvement of the β-catenin signalling pathway in hair follicle development and cycling, all of which highlight the importance of VEGF and β-catenin signalling in minoxidil's hair growth-promoting effects. ⁽⁶⁾

4.Androgen Suppression:

Research suggests that Minoxidil might have a role in blocking the effects of androgens. In lab studies, it's been shown to decrease the activity of 5α-reductase type 2 in human keratinocyte cells. However, other studies have yielded mixed results, with some indicating a potential increase in 5α-reductase activity in dermal papilla cells from balding scalps or no effect in animal models like the golden hamster. As a result, the evidence for Minoxidil's antiandrogenic properties is not yet conclusive. The complexity of this issue is further highlighted by variability in 5α-reductase expression and activity across different cell types and tissues, suggesting that Minoxidil's potential antiandrogenic effects may be dependent on specific cellular or tissue contexts. Ultimately, additional research is needed to clarify the mechanisms underlying Minoxidil's potential antiandrogenic properties and their relevance to hair growth promotion, underscoring the need for further investigation into this multifaceted topic. ⁽⁶⁾

5.Genetic Replication and Cellular growth:

Minoxidil appears to enhance DNA synthesis in the hair follicle bulb, promoting the anagen phase. By stimulating secondary germ cells in telogen follicles, it can potentially accelerate the transition from the telogen phase to the anagen phase, leading to earlier hair growth initiation. This mechanism may be further supported by increased cell proliferation and growth in the hair follicle, enhanced hair follicle development and maintenance, and potential prolongation of the anagen phase, ultimately leading to thicker and longer hair. These factors highlight the potential benefits of Minoxidil's effects on DNA synthesis and hair growth initiation, underscoring its role in promoting hair growth. ⁽⁶⁾

6.Role of Minoxidil in Anagen and Telogen Phase:

Minoxidil may influence hair growth by altering the balance between the anagen and telogen phases of hair follicles. In one study, a topical Minoxidil solution was found to shorten the telogen phase, allowing hair growth to commence earlier. Another study showed that a higher concentration of Minoxidil solution affected both anagen and telogen phases and enlarged hair follicles. In human trials, topical application of a lower concentration of Minoxidil increased the number of hairs in the anagen phase while reducing the number of hairs in the telogen phase. These findings suggest that Minoxidil can promote hair growth by regulating the hair growth cycle. ⁽⁶⁾

7.Autoimmune

Researchers have explored the potential benefits of Minoxidil for treating Alopecia Areata, an autoimmune disorder that causes hair loss. One theory is that applying Minoxidil directly to the scalp may reduce local immunological events, allowing hair to regrow. This potential mechanism is supported by its possible role in modulating cytokine activity to regulate immune responses, enhancing hair follicle survival and reducing inflammation, as well as stimulating hair growth through increased blood flow and nourishment to the hair follicles. Although these findings are promising, the precise impact of Minoxidil on the body's immune system in vivo remains unknown, necessitating further research to fully understand its effects and optimize treatment approaches. ⁽⁶⁾

Effects of Minoxidil on cells:

The hair follicle is a multifaceted structure comprising a diverse array of cell types, including epithelial, dermal, immune, and pigmented cells, as well as a vascular and neural network. The interplay between these cellular components governs their differentiation and, subsequently, the hair cycle. Investigations focusing on mouse hair follicles have revealed that both minoxidil and minoxidil sulphate tend to accumulate in melanocytes and pigmented cells of the suprapapillary region of the follicle. This targeted accumulation may contribute to the mechanism by which minoxidil influences hair growth, highlighting the complex interactions between various cell types in the hair follicle. ⁽¹⁰⁾

1.Synthesis of Collagen:

Investigators discovered that minoxidil suppressed the activity of the enzyme lysyl hydroxylase in human skin fibroblast cultures, resulting in the production of collagen deficient in hydroxylysine. One of the primary steps in collagen synthesis involves the hydroxylation of lysine residues, which is facilitated by a family of lysyl hydroxylases. This hydroxylation process is vital for the formation of enzymatic cross-linking. In fibrotic disorders characterized by excessive collagen accumulation, the increased cross-linking renders collagen more resistant to proteolytic degradation. Minoxidil's impact on collagen production is most pronounced on LH1 mRNA levels, involved in the hydroxylation of triple helical lysine. By reducing the levels of LH1 mRNA, minoxidil decreases lysyl hydroxylase activity, potentially limiting the formation of hydroxyallysine cross-links. Some research suggests that this effect could contribute to minoxidil's potential anti-fibrotic properties. However, other studies, such as those by Zuurmond et al., attribute minoxidil's effects on collagen to alterations in the collagen matrix rather than a reduction in fibrosis. ⁽¹⁰⁾

2.Cytoplasmic Growth:

Research has indicated that minoxidil can have a dual effect on cell growth, promoting proliferation at certain concentrations while inhibiting it at higher levels. Studies have found that elevated concentrations of minoxidil can impede the growth of skin fibroblasts, as demonstrated by Murad and Pinnell. The variability in results from cell culture experiments may be attributed to the presence of aminoglycoside antibiotics, which block potassium channels and are often added to cell culture media, as hypothesized by Sanders et al. In experiments where these aminoglycoside antibiotics were absent, minoxidil was shown to enhance the growth of skin fibroblasts, but this effect was not observed when the antibiotics were present. Furthermore, the aminoglycosides partially diminished the ability of human keratinocytes to respond to minoxidil's growth-promoting effects. ⁽¹⁰⁾

3.Effect on Androgen Receptor:

The mechanism of action of minoxidil on hair growth appears to be distinct from androgen pathways, given its ability to promote hair growth in both androgen-dependent and androgen-independent hair follicles. Nevertheless, research conducted by Sato et al. reveals that minoxidil stimulates the activity of 17 beta-hydroxysteroid dehydrogenase, an enzyme that catalyses the dehydrogenation of 17-hydroxysteroids in steroidogenesis, particularly converting testosterone to androstenedione. Furthermore, minoxidil exhibits a small inductive effect on 5-alpha-reductase, another enzyme involved in androgen metabolism. These findings imply that although minoxidil's primary mechanism may not be directly linked to androgen pathways, it may still exert some influence on androgen metabolism within the hair follicle, particularly in dermal papilla cells. ⁽¹⁰⁾

4.Synthesis of Prostaglandin:

Studies have found that minoxidil enhances the production of specific signalling molecules, including PGE2 and leukotriene B4, in dermal papilla cells, while reducing the production of prostacyclin. This effect may be due to the activation of prostaglandin endoperoxidase synthase-1. Additionally, minoxidil increases the expression of the EP2 receptor, a specific receptor for PGE2, which is the most upregulated target gene of β-catenin signalling in human DPCs. The increased expression of EP2 may contribute to the prolongation of the anagen phase. ⁽¹⁰⁾

5.Vascular effects:

Researchers examined the impact of topical application of minoxidil, at varying concentrations (1%, 3%, and 5%), on blood flow in the balding scalp using laser Doppler velocimetry (LDV) and photo pulse plethysmography. The results indicated an increase in skin blood flow, particularly with the 5% solution. However, another study by Bunker and Dowd found no significant change in skin blood flow after applying a 3% minoxidil solution to the scalp in a group of men, although they did respond to 0.1% hexyl nicotinate, a known vasodilator. A separate investigation by Sakita et al. using electronic microscopy observed changes in the hair follicle vasculature in individuals treated with minoxidil, noting an increase in capillary fenestrations. ⁽¹⁰⁾

6.VEGF:

A research study found that minoxidil enhances the production of VEGF, a protein that promotes the formation of new blood vessels around hair follicles, leading to increased hair growth. VEGF's expression in human dermal papilla cells is boosted by minoxidil in a dose-dependent fashion. The increased perifollicular vascularization is thought to contribute to the accelerated hair growth observed with minoxidil's use. ⁽¹⁰⁾

Pharmacokinetics of Minoxidil:

Absorption:

The body absorbs oral Minoxidil quickly through the gastrointestinal tract, with peak plasma concentrations appearing in the bloodstream within a short time frame, typically within an hour. Topical application of Minoxidil, however, results in much less absorption, with only a small fraction (approximately 1.2-1.4% for 2-3% solutions) of the applied amount entering the bloodstream. Consequently, serum concentrations of Minoxidil are typically very low, often barely detectable, usually below 5 ng/mL. Research has shown that even when applied topically, Minoxidil's presence in the blood can be minimal and variable. Interestingly, the amount absorbed through topical application is estimated to be equivalent to a relatively low oral dose, with two applications of 5% topical Minoxidil predicted to be equivalent to around 5.4mg oral dose. ⁽⁶⁾

Distribution and Metabolism:

The metabolism of Minoxidil into its active sulphated metabolite is critical for its effectiveness, as this metabolite is responsible for its therapeutic benefits. When ingested orally, Minoxidil is primarily metabolized in the liver through glucuronidation, hydroxylation, and sulfation. It has been observed that Minoxidil has minimal binding to plasma proteins and does not cross the blood-brain barrier. In the case of topical Minoxidil application, follicular sulfotransferase plays a crucial role in activating the medication, highlighting the importance of local biochemical processes in its mechanism. ⁽⁶⁾

Excretion:

Minoxidil is cleared from the body fairly rapidly when taken orally, with an elimination half-life of 3-4 hours. The kidneys play a key role in removing it from the body, with most of it being excreted in the urine within 12-20 hours. After topical application is stopped, the body efficiently eliminates the absorbed amount, with approximately 95% of systemically absorbed Minoxidil being cleared within 4 days. ⁽⁶⁾

Androgenetic aloplecia:

Androgenetic alopecia (AGA), also known as male/female pattern baldness, is a common hereditary condition characterized by progressive hair thinning and loss, particularly on the scalp. It affects approximately 50% of men and women by age 50, with varying degrees of severity. The condition is influenced by genetics, hormones, and aging, with dihydrotestosterone (DHT) playing a key role in hair follicle miniaturization. Family history also contributes to the risk, and other factors such as stress, nutritional deficiencies, and certain medical conditions may also play a role. AGA disrupts the normal hair growth cycle, leading to a shorter growth phase and a longer resting phase, resulting in progressive hair thinning and increased shedding. Symptoms typically start at the temples in men and the crown in women, with gradual progression over time. Diagnosis involves clinical examination, medical history evaluation, and sometimes scalp biopsy. While lifestyle modifications such as stress management, a balanced diet, and gentle scalp care may help promote hair growth, understanding the condition early on can help individuals make informed decisions about managing their hair loss. Each person's experience with AGA is unique, and ongoing research may lead to a better understanding of the condition.

Patients:

The clinical trial's methodology involved assessing 70 individuals with mild to moderate androgenetic alopecia (AGA) for study eligibility. Ineligible participants included those who had recently used topical or systemic treatments for AGA, patients with other causes of hair loss such as inflammatory or scarring alopecia, hyperandrogenism, or hormonal disorders, and those with hypersensitivity to minoxidil or severe systemic diseases like renal, cardiovascular, or hepatic issues. Additionally, patients with hypotension/hypertension or those on antihypertensive therapy, as well as pregnant or breastfeeding women, were excluded. One person declined participation, and four others were deemed ineligible. Eligible participants underwent a thorough scalp examination and trichoscopy to rule out other hair loss causes, staging of AGA using the Norwood-Hamilton scale (for males) or Ludwig scale (for females), and various laboratory tests, including haematological, liver and kidney function, thyroid function, electrolytes, and pregnancy tests (for females). Blood pressure and body weight were also monitored during follow-up visits, ensuring a comprehensive evaluation that prioritized safety and accuracy. ⁽¹²⁾

Treatment protocol:

The study involved 65 participants who were randomly assigned into two groups based on a block randomization strategy. One group received a daily dose of 1 mg oral minoxidil, while the other group applied topical minoxidil 5% (with men using 1 cc twice daily and women using 1 cc once daily). The treatment lasted for 6 months, during which patients were required to return for assessments at 3 and 6 months after treatment initiation. The medications used in the study were prepared under the supervision of pharmaceutical consultants at the University Pharmacy Faculty laboratory. ⁽¹²⁾