Few drugs have travelled a stranger path than minoxidil. Synthesised in the 1970s as an oral antihypertensive for patients whose blood pressure refused to respond to anything else, it quickly became notorious for an unusual side effect: the people taking it kept growing more hair. What at first looked like a cosmetic nuisance turned out to be the most commercially and clinically significant observation the molecule would ever produce. Within a decade, minoxidil had been reformulated as a topical solution, approved for male pattern hair loss in 1988 and for female pattern hair loss in 1992, and positioned as the first non-surgical, FDA-approved treatment for androgenetic alopecia.
For years, the standard explanation for how topical minoxidil worked was satisfyingly simple. The drug was a vasodilator, so it must have been improving blood flow to the scalp, bringing more oxygen and nutrients to hair follicles and nudging them back into the growth phase. That account held up neatly during the 1990s and early 2000s, and it became the line most clinicians used when explaining the drug to patients. The only problem was that it never quite explained everything. Response rates stubbornly sat around 30–40%, systemic absorption from topical use was minimal, and laboratory studies kept uncovering effects on hair follicle cells that had nothing to do with blood vessels at all.
Table of contents
- Introduction
- Outline of article
- I. Historical Development and Serendipitous Discovery
- II. The Vasodilation Hypothesis: Initial Understanding of Minoxidil’s Mechanism
- III. The Prodrug Revolution: SULT1A1 and Bioconversion
- IV. Poor Responders: Genetic and Enzymatic Determinants
- V. Beyond Vasodilation: Molecular Signalling and Direct Cellular Effects
- VI. Integrated Modern Understanding and Future Perspectives
- Conclusion
- References:
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Introduction
The picture that has emerged over the last two decades is considerably more interesting. Minoxidil is now recognised as a prodrug that must be activated in the hair follicle itself, through sulfation by the enzyme SULT1A1 in the outer root sheath. Individual variation in this enzyme’s activity has turned out to explain, at a molecular level, why some patients respond dramatically to minoxidil while others see almost nothing, and has opened the door to practical pharmacogenetic testing and enzyme-boosting combination therapies.
At the same time, cellular and molecular research has shown that the active metabolite does far more than relax blood vessels. It stimulates the secretion of growth factors including VEGF, FGF, and PDGF, activates Wnt/beta-catenin signalling, protects dermal papilla cells from apoptosis, and modulates inflammatory cytokines such as IL-1alpha that are known to suppress follicle growth.
This review traces that story in full. It begins with the serendipitous discovery of minoxidil’s hair-growth effect and the era of the vasodilation hypothesis, then examines the prodrug revolution brought about by SULT1A1, the genetic and enzymatic reasons why some patients respond poorly, and the growing body of work on the drug’s direct cellular and immunological effects.
It closes by looking at how these insights have reshaped modern practice, from combinatorial therapies and genetic screening to new delivery systems and emerging applications beyond androgenetic alopecia. The broader aim is to show how a drug originally intended for one organ became a tool for understanding the biology of an entirely different one, and how the mechanistic picture has shifted from a simple story about blood flow to a layered model of hair follicle pharmacology that is still being written.
Outline of article
I. Historical Development and Serendipitous Discovery
- A. Initial Development as Oral Antihypertensive (1970s)
- B. Discovery of Hypertrichosis Side Effect
- C. Repurposing and Topical Formulation Development
- D. FDA Approval Timeline and Clinical Implementation
II. The Vasodilation Hypothesis: Initial Understanding
- A. Original Mechanism of Action Theory
- B. Blood Flow Enhancement and Follicular Nutrient Delivery
- C. Potassium Channel Opening Properties
- D. Early Clinical Evidence Supporting Vasodilation Theory
III. The Prodrug Revolution: SULT1A1 and Bioconversion
- A. Minoxidil as a Prodrug: The Sulfotransferase Discovery
- B. SULT1A1 Enzyme: Structure, Function, and Scalp Expression
- C. Conversion to Minoxidil Sulfate: The Active Metabolite
- D. Bioconversion Efficiency and Clinical Relevance
IV. Poor Responders: Genetic and Enzymatic Determinants
- A. The Mystery of Variable Response Rates (30-40% Efficacy)
- B. SULT1A1 Activity as a Predictive Biomarker
- C. Genetic Polymorphisms and Ethnic Variations
- D. Emerging Strategies to Convert Non-Responders to Responders
V. Beyond Vasodilation: Molecular Signalling and Direct Cellular Effects
- A. Growth Factor Stimulation and Angiogenesis (VEGF, FGF, PDGF)
- B. Wnt/beta-catenin Signalling Pathway Activation
- C. Dermal Papilla Cell Proliferation and Anti-Apoptotic Effects
- D. Immune and Inflammatory Pathway Modulation
VI. Integrated Modern Understanding and Future Perspectives
- A. Multifactorial Mechanism Model
- B. Combinatorial Therapies and Enhanced Efficacy
- C. Personalised Medicine and Genetic Screening
- D. Emerging Delivery Systems and Novel Applications
I. Historical Development and Serendipitous Discovery
A. Initial Development as an Oral Antihypertensive Agent
Minoxidil was initially synthesised in the 1970s as an orally administered medication specifically designed to treat severe, refractory hypertension [1]. The drug was developed during an era when effective treatment options for patients with hypertension resistant to standard multidrug regimens were severely limited. Its vasodilatory properties made it an attractive candidate for systemic hypertension management, particularly in patients who had failed conventional therapies. The pharmacological profile of minoxidil as a potassium channel opener demonstrated significant efficacy in reducing blood pressure in this challenging patient population [2]. However, despite its therapeutic benefits for hypertension management, minoxidil has remained largely restricted to specialised clinical use due to its significant systemic adverse effects, particularly tachycardia and fluid retention, which limit its widespread adoption.
B. Discovery of Hypertrichosis as a Clinical Side Effect
The most consequential discovery in minoxidil’s trajectory came with the observation of hypertrichosis—excessive hair growth—as a notable adverse effect in hypertensive patients receiving the drug [3]. This side effect, initially regarded as an unwanted complication of minoxidil therapy, proved to be far more valuable than the drug’s original indication. Clinicians treating hypertensive patients with minoxidil began noticing increased hair growth in various body areas, suggesting that the drug possessed previously unrecognised hair-growth-promoting properties. This observation of hypertrichosis marked a pivotal moment in dermatological pharmacology, shifting the focus from the drug’s systemic cardiovascular effects to its potential topical use for treating androgenetic alopecia [4]. The hypertrichosis finding demonstrated that minoxidil could modulate hair follicle biology at both systemic and local levels, opening an entirely new therapeutic avenue.
C. Repurposing and Development of Topical Formulations
Following the recognition of minoxidil’s hair-growth-promoting effects, pharmaceutical companies pursued the development of topical formulations to harness these benefits while minimising systemic adverse effects. This drug repurposing effort represented a successful example of how unexpected side effects can lead to therapeutic innovations [4]. The development of topical minoxidil solutions was driven by the recognition that the drug could be applied directly to areas of hair loss on the scalp, potentially delivering effective concentrations to hair follicles while reducing systemic absorption and associated cardiovascular complications. Two primary formulations were developed and eventually approved: a 2% solution for female androgenetic alopecia and a 5% solution for male androgenetic alopecia [3]. The topical approach proved more practical for chronic hair loss treatment, as it allowed patients to apply the medication consistently over extended periods without the systemic side effects associated with oral administration.
D. FDA Approval Timeline and Clinical Implementation
Minoxidil became the first topically applied medication FDA-approved for the treatment of androgenetic alopecia. The 2% topical formulation received FDA approval in 1988 for the treatment of male pattern baldness and in 1992 for the treatment of female pattern hair loss [1]. This regulatory milestone established topical minoxidil as a first-line treatment option for androgenetic alopecia and represented the only non-surgical therapeutic option available at the time. Subsequent FDA approvals expanded the minoxidil portfolio to include additional formulations and concentrations, including a 5% topical solution and foam preparations [2]. The regulatory pathway for minoxidil served as a template for subsequent drug repositioning efforts in dermatology. Despite its early regulatory approval and widespread availability, minoxidil’s mechanism of action remained incompletely understood for decades, prompting intensive basic and clinical research to elucidate how this drug promotes hair growth at the molecular and cellular levels [5].
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II. The Vasodilation Hypothesis: Initial Understanding of Minoxidil’s Mechanism
A. The Original Mechanism of Action Theory
For nearly two decades after the introduction of topical minoxidil, the predominant explanation for its hair-growth-promoting effect was its vasodilatory properties. The vasodilation hypothesis proposed that minoxidil improved blood flow to hair follicles through its action as a vasodilator, thereby increasing the delivery of oxygen, nutrients, and growth factors to the follicular bulb and dermal papilla [3]. This mechanistic framework appeared logical given minoxidil’s original development as a vasodilator for hypertension, and the observation that improved vascular perfusion could, in theory, support follicular growth. The vasodilation theory dominated the minoxidil literature through the 1990s and early 2000s, shaping clinical practice guidelines and patient education materials [4]. Clinicians explained to patients that minoxidil “worked by increasing blood flow to the scalp,” a concept readily understood and accepted by the general public.
B. Potassium Channel Opening and Blood Flow Enhancement
The cellular basis for minoxidil’s vasodilatory effects involves its action as a potassium channel opener. Minoxidil functions by opening ATP-sensitive potassium channels in vascular smooth muscle cells, leading to hyperpolarisation of the cell membrane and subsequent smooth muscle relaxation [2]. This mechanism of action is shared with other potassium channel openers used in cardiovascular medicine. In the context of hair follicles, the activation of potassium channels in dermal vasculature and perifollicular vessels was hypothesised to produce local vasodilation, enhancing microvascular blood flow to hair follicles. The improved blood supply was thought to deliver increased quantities of oxygen and nutrients—including amino acids, glucose, and cofactors necessary for hair synthesis—directly to the hair follicle [6]. Additionally, the enhanced vascular perfusion could potentially promote the delivery of circulating growth factors and removal of metabolic waste products from the follicular microenvironment, potentially improving the overall metabolic state of hair-producing cells.
C. Follicular Nutrient Delivery and Metabolic Support
Dermal papilla cells, which are central regulators of hair follicle function, depend critically on adequate blood supply for maintaining their metabolic activity and secretion of growth-promoting factors. The vasodilation hypothesis proposed that increased blood flow enhanced nutrient delivery to these critical cells, supporting their proliferative capacity and growth factor synthesis. Hair follicle miniaturisation, a hallmark of androgenetic alopecia, involves progressive reductions in hair shaft diameter and follicular size, which has been associated with impaired microvasculature and reduced oxygen delivery to follicles [7]. By improving vascular perfusion, minoxidil could theoretically reverse the microvascular insufficiency associated with miniaturised follicles and support their transition from the telogen (resting) phase to the anagen (growth) phase. The nutrient-delivery hypothesis seemed particularly compelling in explaining why topical minoxidil could affect hair follicles across different regions of the scalp, suggesting a systemic effect mediated by blood flow.
D. Early Clinical Evidence and Limitations of the Vasodilation Theory
Early clinical trials of topical minoxidil documented its efficacy in promoting hair regrowth in both men and women with androgenetic alopecia, providing empirical support for its therapeutic use [2]. Meta-analyses of 2% minoxidil efficacy indicated that the treatment produced hair regrowth in 30-40% of patients in controlled trials, with peak effects typically observed after 4-6 months of continuous application [2]. These clinical findings appeared consistent with the vasodilation mechanism, as improved blood flow could be expected to promote hair growth. However, several observations began to challenge the sufficiency of the vasodilation hypothesis in explaining minoxidil’s full pharmacological profile. First, systemic absorption of topical minoxidil is minimal (approximately 1.4% of the applied dose) [2], raising questions about whether systemic vasodilation could account for topical effects. Second, the variable response rates across patients—with some individuals showing robust hair regrowth while others demonstrated minimal response despite similar application regimens—were difficult to explain based purely on vasodilatory mechanisms. Third, emerging molecular studies revealed that minoxidil’s effects on hair follicle cells extended well beyond simple improvements in blood flow, suggesting additional, direct cellular mechanisms of action.
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III. The Prodrug Revolution: SULT1A1 and Bioconversion
A. Minoxidil as a Prodrug: The Sulfotransferase Enzyme Discovery
A transformative breakthrough in minoxidil research came with the recognition that minoxidil functions as a prodrug requiring metabolic activation to exert its biological effects. Unlike many therapeutic agents that are active in their administered form, minoxidil is biologically inert and must undergo enzymatic conversion to become pharmacologically active [1]. This critical discovery fundamentally changed the understanding of minoxidil’s mechanism of action and explained many previously puzzling clinical observations. The identification of sulfotransferase enzymes as responsible for minoxidil’s bioconversion represented a major milestone in hair growth pharmacology [8]. Specifically, the enzyme sulfotransferase 1A1 (SULT1A1), belonging to the phase II detoxification enzyme family, catalyses the transfer of a sulfonate group from the universal sulfate donor 3′-phosphoadenosine 5′-phosphosulfate (PAPS) to minoxidil, generating minoxidil sulfate [9]. This prodrug-to-active-drug conversion occurs primarily in the outer root sheath of hair follicles, with sulfotransferase activity varying substantially among individuals [2].
B. SULT1A1 Enzyme: Structure, Function, and Scalp Expression
SULT1A1 belongs to a superfamily of cytoplasmic sulfotransferases that catalyse the sulfation of a wide variety of endogenous and xenobiotic compounds, functioning as a phase II xenobiotic-metabolising enzyme [10]. These enzymes are expressed in many tissues, with particularly high levels in the liver and gastrointestinal tract, but they are also present in skin tissues, including the scalp and hair follicles [9]. The tissue-specific and cellular-specific expression patterns of SULT1A1 have important implications for minoxidil bioavailability and efficacy. In the scalp, SULT1A1 is expressed in the outer root sheath epithelium of hair follicles, where it catalyses minoxidil sulfation [10]. The enzyme requires PAPS as a sulfate donor, and its activity is regulated by substrate availability, cofactor levels, and potentially by hormonal and inflammatory factors. Individual variations in SULT1A1 expression and activity levels have been identified across different populations, suggesting both genetic and environmental factors contribute to enzyme expression [11]. These variations in enzyme activity have profound clinical implications for minoxidil response, as will be discussed in subsequent sections.
C. Conversion to Minoxidil Sulfate: The Active Metabolite
The bioconversion of minoxidil to minoxidil sulfate represents the critical step in the drug’s activation and the initiation of its pharmacological effects on hair follicles. Minoxidil sulfate, the active metabolite, differs from the parent compound in its chemical structure, pharmacological properties, and biological activities [8]. This sulfation-mediated activation is not merely a detoxification or elimination process but rather represents an activation mechanism essential for therapeutic efficacy. Following topical application of minoxidil to the scalp, the drug must first penetrate the stratum corneum and reach the hair follicle, where it encounters SULT1A1-expressing cells in the outer root sheath. Once bioconverted to minoxidil sulfate, the active metabolite can interact with its molecular targets within hair follicle cells [10]. The discovery that minoxidil sulfate, rather than the parent minoxidil molecule, is the active species fundamentally altered the pharmacological understanding of minoxidil’s effects on hair follicles. Importantly, the efficiency of this bioconversion varies significantly among individuals due to differences in SULT1A1 expression and activity, establishing a direct link between enzyme function and clinical response.
D. Bioconversion Efficiency and Clinical Relevance
The efficiency of minoxidil-to-minoxidil sulfate conversion directly determines the concentration of active drug available to interact with hair follicle target cells. Individuals with high SULT1A1 activity in hair follicles achieve greater bioconversion of topical minoxidil, resulting in higher local concentrations of the active metabolite and, correspondingly, stronger pharmacological effects [1]. Conversely, individuals with low follicular SULT1A1 activity produce less minoxidil sulfate and may achieve subtherapeutic concentrations of the active metabolite, resulting in minimal or absent hair growth promotion despite consistent topical application [8]. This mechanistic understanding explains the previously puzzling observation that clinical response to minoxidil is highly variable, with approximately 30-40% of patients showing significant hair regrowth while 50-60% demonstrate minimal response [9]. The bioconversion efficiency model provided a rational, molecular-level explanation for clinical non-response, transforming it from an enigmatic observation into an understandable consequence of individual differences in enzyme activity. Furthermore, the identification of bioconversion as the rate-limiting step in minoxidil efficacy opened possibilities for enhancing treatment response through strategies designed to increase SULT1A1 activity or expression, as discussed in subsequent sections.
IV. Poor Responders: Genetic and Enzymatic Determinants
A. The Challenge of Variable Response Rates and the Definition of Poor Responders
Despite decades of clinical use, minoxidil demonstrates limited efficacy in a substantial proportion of patients receiving the medication. Clinical trials and meta-analyses consistently document that only 30-40% of patients with androgenetic alopecia achieve clinically significant hair regrowth with topical minoxidil monotherapy [8], while 50-60% show minimal or no response despite months of consistent application. This division of patients into responders and non-responders has become a central concern in alopecia management, as it limits the ability to provide effective treatment to nearly half of patients seeking hair loss therapy [12]. The term “poor responder” has been formally defined in the literature as patients showing minimal hair growth response despite 3-6 months of consistent minoxidil application at standard therapeutic concentrations. The persistence of a substantial population of poor responders has prompted extensive investigation into the biological factors determining response variability, with the SULT1A1 enzyme emerging as a critical determinant [11].
B. SULT1A1 Activity as a Predictive Biomarker for Clinical Response
Landmark research by Goren and colleagues established that follicular SULT1A1 enzymatic activity serves as a powerful predictor of clinical response to topical minoxidil [8]. Their initial studies demonstrated a correlation between sulfotransferase activity measured in plucked hair follicles and subsequent clinical response to minoxidil treatment, with a sensitivity of 95% and specificity of 73% in predicting responders [8]. Subsequent investigations replicated and extended these findings, establishing that SULT1A1 activity testing could effectively identify non-responders to minoxidil prior to initiating therapy, potentially avoiding unnecessary months of treatment in patients unlikely to benefit [13]. Studies specifically evaluating sulfotransferase activity in female androgenetic alopecia patients demonstrated 93% sensitivity and 83% specificity in predicting treatment response [14]. These high predictive values have established SULT1A1 activity measurement as a potentially valuable clinical tool for patient stratification and treatment planning [11]. Patients with high follicular SULT1A1 activity demonstrate hair regrowth rates of 60-70%, while those with low enzyme activity achieve response rates of only 10-15%, highlighting the dramatic influence of bioconversion efficiency on clinical outcome.
C. Genetic Polymorphisms and Ethnic Variations in SULT1A1 Expression
The substantial inter-individual variation in SULT1A1 activity reflects underlying genetic diversity in the SULT1A1 gene and potentially in genes regulating its expression. Single-nucleotide polymorphisms (SNPs) in the SULT1A1 gene have been identified as factors influencing enzyme activity and expression levels [15]. Notably, a specific SNP (rs1042028) in the SULT1A1 gene has been identified as a robust predictor of poor response to minoxidil across multiple studies [15]. This polymorphism was found to be associated with poor response to minoxidil with statistical significance of p = 2.4 × 10^-8 and an odds ratio of 0.09, indicating a strong protective effect of the favourable genotype [15]. Furthermore, the prevalence of low SULT1A1 activity varies across ethnic populations. Research examining follicular sulfotransferase activity in pattern hair loss patients from the Indian subcontinent found that 40.8% had low enzyme levels, with notably higher prevalence in men (49.3%) compared to women (26.6%) [16]. These ethnic variations in SULT1A1 activity and SULT1A1 SNP frequencies have important implications for understanding why minoxidil efficacy rates vary across different populations worldwide. Genetic factors contributing to SULT1A1 expression appear to play a substantial role in determining baseline enzyme activity and, consequently, individual likelihood of responding to topical minoxidil therapy.
D. Strategies to Convert Poor Responders and Enhance Minoxidil Efficacy
The recognition that poor response to minoxidil is predominantly determined by low SULT1A1 activity has prompted investigation into strategies designed to enhance follicular SULT1A1 expression or activity, thereby converting non-responders into responders. One promising approach involves the use of topical tretinoin (all-trans retinoic acid), a retinoid known to upregulate various metabolic enzymes [17]. A landmark study demonstrated that topical tretinoin application influenced SULT1A1 expression in hair follicles, with remarkably 43% of subjects initially predicted to be minoxidil non-responders based on baseline SULT1A1 activity being converted to responders following 5 days of tretinoin pretreatment [17]. This finding has important clinical implications, suggesting that combination therapy with tretinoin and minoxidil may improve overall treatment efficacy. More recently, novel SULT1A1 enzyme booster formulations have been developed, specifically designed to increase follicular SULT1A1 activity. Clinical trials of these enzyme booster solutions combined with minoxidil demonstrated that 65% of patients who were non-responders to minoxidil monotherapy exhibited a positive response to the combination therapy [12]. These emerging strategies offer hope to the substantial population of poor responders, potentially allowing extension of effective minoxidil treatment to previously treatment-resistant patients.
V. Beyond Vasodilation: Molecular Signalling and Direct Cellular Effects
A. Growth Factor Stimulation and Angiogenesis: VEGF, FGF, and PDGF
While minoxidil’s initial recognition as a vasodilator focused attention on improved blood flow as its mechanism of action, subsequent molecular and cellular research revealed that minoxidil exerts direct, blood-flow-independent effects on hair follicle cells through stimulation of growth factor synthesis and angiogenic signalling. Minoxidil has been demonstrated to enhance vascular endothelial growth factor (VEGF) expression in dermal papilla cells and other follicular cell types [18]. VEGF is a critical mediator of angiogenesis that promotes the formation of new blood vessels around hair follicles, supporting follicle nutrient delivery through mechanisms extending beyond simple vasodilation [18]. Importantly, oral minoxidil administration has been shown to increase serum VEGF levels in patients with androgenetic alopecia, with statistically significant increases correlating with improvements in hair count and reductions in hair shedding [19]. Beyond VEGF, minoxidil stimulates adipose-derived stem cells to increase secretion of additional growth factors, including fibroblast growth factor (FGF), platelet-derived growth factor (PDGF), and chemokine (C-X-C motif) ligand 1 (CXCL1) [18]. These growth factors, when secreted into the local follicular microenvironment, enhance dermal papilla cell proliferation and support hair follicle growth through multiple signalling pathways. The finding that minoxidil promotes growth factor secretion even in the absence of improved vasodilation indicates that growth factor stimulation represents a distinct, direct mechanism contributing to minoxidil’s hair-growth-promoting effects.
B. Wnt/beta-catenin Signalling Pathway Activation
Among the most significant discoveries regarding minoxidil’s mechanism of action beyond vasodilation is its ability to activate Wnt/beta-catenin signalling, a critical developmental and regenerative pathway in hair follicle biology. The Wnt/beta-catenin pathway plays essential roles in hair follicle development, the hair cycle, and follicle regeneration through regulation of stem cell activity and differentiation [20]. Activation of this pathway promotes the telogen-to-anagen transition, extending the growth phase and promoting follicular proliferation. Multiple studies have demonstrated that minoxidil upregulates components of the Wnt/beta-catenin signalling cascade in dermal papilla cells and follicular tissue, including increased expression of Wnt family proteins, ?-catenin, and lymphoid-enhancer-binding factor 1 (LEF1) [21]. This activation occurs both in vitro in cultured follicular cells and in vivo in animal models of hair growth [22]. The mechanisms through which minoxidil activates Wnt/beta-catenin signalling appear to involve both direct effects on Wnt ligand expression and indirect effects on the pathway through modification of microenvironmental factors. Notably, Wnt/beta-catenin pathway activation appears to be independent of minoxidil’s vasodilatory effects, representing a distinct mechanism contributing to hair growth promotion [20].
C. Dermal Papilla Cell Proliferation and Anti-Apoptotic Effects
Direct examination of minoxidil’s effects on dermal papilla cells, the critical mesenchymal component of hair follicles responsible for growth factor secretion and follicle induction, has revealed multiple pro-proliferative and anti-apoptotic effects. Minoxidil treatment enhances dermal papilla cell proliferation in vitro, with increased expression of proliferation markers including Ki-67 and cyclin proteins [18]. Additionally, minoxidil changes the activity of several key “cell control” signals in lab-grown human dermal papilla cells (cells that help drive hair growth).
The researchers found that minoxidil switches on two pathways linked with growth and survival (ERK and Akt). That boost helps the cells stay alive and shifts the balance of signals away from cell death, by increasing the Bcl-2 to Bax ratio.
In practical terms, the authors suggest minoxidil may support hair growth by helping these follicle cells survive and multiply, which could keep hairs in the anagen (growth) phase for longer [23].
The anti-apoptotic effects of minoxidil are particularly relevant in the context of androgenetic alopecia, wherein dihydrotestosterone (DHT) triggers apoptosis of dermal papilla cells, contributing to follicle miniaturisation [23]. By promoting dermal papilla cell survival and proliferation, minoxidil may counteract DHT-induced cellular damage and support the maintenance of follicular function. These direct cellular effects on dermal papilla biology represent mechanisms through which minoxidil promotes hair growth independent of any systemic or local vasodilatory effects, fundamentally broadening the understanding of the drug’s pharmacological profile.
D. Immune and Inflammatory Pathway Modulation
The pilosebaceous unit has long been recognised as a site of extensive cell and tissue interaction. A range of cell types has been described as being involved, including epithelial cells, the sebaceous gland, fibroblasts, the dermal papilla, melanocytes, endothelial cells, and Langerhans cells of the immune system. Although the histology and overall structure of the hair follicle had been well characterised, the signalling that governs it has continued to be refined over time. Through in vitro and in vivo models, the contributions of growth factors such as VEGF, insulin-like growth factor-1, and transforming growth factor alpha were clarified, along with the roles of androgens, including dihydrotestosterone, and cytokines such as IL-1. Even so, the coordinated signalling behind follicular development and the hair cycle has not yet been fully explained [24].
Androgenetic alopecia (AGA) and alopecia areata (AA) have both been described as conditions that target the hair follicle, carry a genetic predisposition, and involve inflammatory mechanisms in their pathogenesis [25]. In AGA, scalp biopsies obtained from both men and women have been shown to exhibit follicular microinflammation and lymphocytic folliculitis, features suggestive of an immunologically driven trigger [25]. That low-grade, ongoing inflammation, combined with connective tissue remodelling, has been proposed as a possible cofactor in the development of permanent hair loss in AGA [26]. AA, by contrast, has been characterised as a T-cell-driven autoimmune disease of the follicle, in which focal inflammatory lesions and perifollicular T-cell infiltrates have been consistently reported [25].
Healthy human epidermis has been identified as a reliable source of biologically active IL-1alpha. Keratinocytes have been shown not only to synthesise this cytokine but also to respond to it via surface receptors, suggesting a meaningful role for the IL-1 system in both epidermal physiology and inflammation [25]. Under normal conditions, IL-1alpha has been reported to contribute to cell growth and repair across a range of epithelial cells and keratinocytes. During inflammation, injury, immunological challenge, or infection, however, its production has been observed to rise sharply, and the resulting biological effects have been shown to contribute to disease [25]. Although IL-1alpha has been described as prominent during skin wounding and inflammatory responses, its expression has also been reported to be downregulated during the anagen phase of the hair cycle [25]. At higher concentrations, IL-1alpha has been shown to exert a rapid antiproliferative effect on hair follicles and has been described as a potent secondary inhibitor of follicle growth [25]. Serum IL-1alpha levels have also been reported to be significantly elevated in patients with the localised form of AA [27]. Despite the evidence implicating inflammation and cytokines in both AGA and AA, the way minoxidil, a drug used in both conditions, interacts with the inflammatory process has not yet been fully defined. Suppressive effects of minoxidil on lymphocyte-mediated immune responses have been reported,[28] but the cytokine-level detail has remained unclear. Among the IL-1 family, IL-1? has been identified as the principal inhibitor of hair growth [25].
Minoxidil has been shown to cause a significant downregulation of IL-1alpha gene expression in HaCaT cells compared with untreated cells. Given the established role of inflammation in both AGA and AA, the clinical efficacy of minoxidil in each, and the inhibition of IL-1alpha expression that has been observed, this anti-inflammatory action has been proposed as one possible mechanism by which the drug acts [25].
Minoxidil itself has been described as a prodrug. Its active form, minoxidil sulfate, has been reported to be generated by minoxidil sulfotransferase, an enzyme that has been detected in several tissues, including the skin. Within skin, sulfotransferase activity has been found to be concentrated in epithelial structures such as proliferating keratinocytes and the outer root sheath of the follicle, rather than in mesenchymal cells [29]. Follicular sulfotransferase activity has also been suggested as a predictor of how well a patient may respond to topical minoxidil, and a sulfotransferase test has been described as a practical way to identify likely non-responders to topical minoxidil therapy in AGA [8]. In view of IL-1alpha’s inhibitory role in human hair growth, it has previously been suggested that identifying the “inflammatory alopecic individual” could be clinically valuable, since anti-IL-1 strategies would be most likely to benefit this subgroup in AGA [30]. Against this background, the anti-IL-1alpha effect that has been demonstrated for minoxidil has been interpreted as another potentially useful approach for flagging likely minoxidil non-responders before treatment is started.
The suppression of pro-inflammatory signalling and the promotion of a more favourable immune microenvironment around follicles represent another mechanism by which minoxidil promotes hair growth independent of vasodilation, further demonstrating the multifactorial nature of the drug’s effects on hair follicle biology.
VI. Integrated Modern Understanding and Future Perspectives
A. Multifactorial Mechanism Model: Integration of Vasodilation, Growth Factors, and Signaling
Contemporary understanding of minoxidil’s mechanism of action recognises that the drug acts through multiple, complementary pathways rather than through a single primary mechanism [2]. The initial vasodilation hypothesis, while containing an element of truth, represents only one component of minoxidil’s multifaceted effects on hair follicles. A comprehensive mechanistic model integrates minoxidil’s vasodilatory effects on perifollicular vasculature with direct cellular effects on follicular components. This integrated model proposes that minoxidil acts through: (1) potassium channel opening in vascular smooth muscle, leading to improved blood flow and nutrient delivery; (2) bioconversion to minoxidil sulfate by follicular SULT1A1, generating the active metabolite responsible for direct cellular effects; (3) stimulation of growth factor synthesis and secretion by dermal papilla cells and adipose-derived stem cells; (4) activation of Wnt/beta-catenin and other growth-promoting signaling pathways; (5) promotion of dermal papilla cell proliferation and suppression of apoptosis; and (6) modulation of inflammatory and immune responses in the follicular microenvironment [2]. These mechanisms are not mutually exclusive but rather operate synergistically to promote the transition of follicles from the resting telogen phase to the active anagen growth phase. The relative contribution of each mechanism may vary among individual patients, potentially explaining the heterogeneous clinical response patterns observed across the patient population [18].
B. Combinatorial Therapies and Enhanced Efficacy
Recognition of minoxidil’s multiple mechanisms of action has prompted investigation into combination therapies designed to enhance overall efficacy by targeting complementary pathways or overcoming SULT1A1-dependent limitations. The combination of minoxidil with tretinoin has demonstrated enhanced efficacy compared to monotherapy, with the retinoid increasing follicular SULT1A1 expression and presumably enhancing bioconversion of topical minoxidil [17]. Combination of minoxidil with low-dose oral finasteride, a 5-alpha reductase inhibitor that blocks the conversion of testosterone to DHT, has shown superior efficacy to either agent alone in some patient populations [2]. Additionally, emerging research has examined combinations of minoxidil with Wnt pathway activators or other agents targeting growth-promoting signalling cascades. For example, research combining minoxidil with exosomes derived from stem cells or platelet-rich plasma has demonstrated enhanced hair follicle activation compared to minoxidil monotherapy [31]. Novel carrier systems for minoxidil delivery, including liposomal formulations and nanostructured systems, have been designed to enhance drug penetration to hair follicles, improve local minoxidil sulfate production, and potentially enhance clinical efficacy [32]. These combinatorial approaches represent the next frontier in minoxidil-based therapeutics, with the potential to extend effective treatment to previously treatment-resistant populations and enhance response rates in minoxidil-responsive patients.
C. Personalised Medicine and Genetic Screening Approaches
The identification of SULT1A1 polymorphisms and other genetic factors determining minoxidil response has laid the foundation for personalised medicine approaches to alopecia treatment. Genetic screening panels capable of identifying SNPs associated with minoxidil, finasteride, and dutasteride response have been developed and are increasingly utilised in clinical practice [15]. A 26-SNP panel was reported to achieve response prediction accuracy rates of 85.6-91.0% for androgenetic alopecia therapeutics, substantially exceeding published benchmark efficacy rates for individual agents [15]. This level of predictive accuracy enables clinicians to select treatments most likely to be effective for individual patients based on their unique genetic profile prior to therapy initiation. Additionally, pharmacogenetic testing can identify patients likely to be poor responders to standard topical minoxidil, potentially guiding selection of alternative therapies such as oral minoxidil, finasteride, dutasteride, or other emerging treatments [15]. As the cost of genetic testing continues to decline and the predictive panels continue to improve, pharmacogenetic-guided treatment selection for alopecia is likely to become increasingly integrated into clinical practice, fundamentally altering the approach to hair loss management from empirical trial-and-error to rationally-informed, personalised treatment selection [15].
D. Emerging Delivery Systems and Expansion of Therapeutic Applications
Beyond improvements in understanding of minoxidil’s mechanisms and optimisation of combination therapies, considerable research effort has focused on developing improved drug delivery systems capable of enhancing minoxidil bioavailability in hair follicles while reducing skin irritation and systemic absorption. Liposomal minoxidil formulations have been demonstrated to enhance both follicular retention and targeted delivery while reducing irritation [32]. The optimised liposomal minoxidil sulfate preparation achieved an average vesicle size of 129.46 nm with 92.72% encapsulation efficiency [32], substantially improving drug delivery compared to conventional minoxidil solutions. Nanostructured minoxidil formulations have similarly shown promise in enhancing follicular penetration and improving bioavailability [33]. Novel carrier systems utilising natural clay minerals and other biocompatible materials have been developed to reduce solvent-induced irritation while maintaining therapeutic efficacy [34]. Ethosomal formulations combining minoxidil with antioxidant compounds have been designed to address oxidative stress in the follicular microenvironment while enhancing minoxidil delivery [35]. Additionally, the finding that minoxidil promotes nail growth through mechanisms similar to those involved in hair growth has prompted exploration of minoxidil applications beyond androgenetic alopecia [36]. Emerging evidence also suggests potential roles for minoxidil in treating alopecia areata and other forms of non-scarring alopecia [37]. These advances in delivery technology and therapeutic expansion represent the continuing evolution of minoxidil-based therapeutics, offering potential for improved patient outcomes and extension of effective treatment to broader populations.
Conclusion
The journey from minoxidil’s serendipitous discovery as a hair-growth-promoting side effect of a hypertension medication to the current understanding of its complex, multifactorial mechanisms of action exemplifies the importance of careful clinical observation and basic scientific investigation in drug development. The evolution of mechanistic understanding—from the initial vasodilation hypothesis through recognition of minoxidil as a prodrug requiring SULT1A1-mediated bioconversion, to appreciation of its direct effects on Wnt/beta-catenin signalling, growth factor stimulation, and immune modulation—demonstrates how scientific understanding of pharmaceutical agents continues to deepen over decades of investigation. The identification of SULT1A1 as a critical determinant of clinical response has provided both a molecular explanation for the previously puzzling variability in minoxidil efficacy and a foundation for the development of personalised treatment strategies and novel approaches to enhance treatment outcomes in poor responders. Contemporary minoxidil research continues to evolve, with emerging investigations into combination therapies, improved delivery systems, and expansion of therapeutic applications promising to further enhance the effectiveness of this important alopecia treatment. As the field moves toward increasingly personalised, mechanistically-informed approaches to hair loss management, minoxidil remains a central therapeutic agent whose continued investigation promises to yield further insights into hair follicle biology and regenerative medicine more broadly.
References:
[15] H. Gaboardi, V. Russo, L. Vila-Vecilla, V. Patel, and G. T. D. Souza, “26-SNP Panel Aids Guiding Androgenetic Alopecia Therapy and Provides Insight into Mechanisms of Action,” Multidisciplinary Digital Publishing Institute, Sep. 2025, https://www.mdpi.com/2079-9284/12/5/190
