Crisaborole is a topical anti-inflammatory drug that works by blocking an enzyme called phosphodiesterase 4, or PDE4, inside skin cells. By shutting down PDE4, crisaborole allows a key signaling molecule (cyclic AMP, or cAMP) to accumulate, which dials down the production of inflammatory chemicals that drive the redness, swelling, and itch of atopic dermatitis. What makes crisaborole unusual among topical treatments is the boron atom at the center of its molecular structure, which gives it both its selectivity for PDE4 and its ability to slip through the skin barrier efficiently.
What PDE4 Does and Why Blocking It Matters
PDE4 is an enzyme found primarily in immune cells involved in inflammatory responses. Its normal job is to break down cAMP, a molecule that acts as a messenger inside cells. When cAMP levels are high, immune cells tend to stay relatively calm. When PDE4 chews through cAMP quickly, those levels drop, and immune cells become more active, releasing inflammatory signals. In atopic dermatitis, PDE4 activity is abnormally high, which tips the balance toward chronic inflammation and flare-ups.
Crisaborole binds to PDE4 and prevents it from degrading cAMP. The result is that cAMP accumulates inside the cell, which activates a downstream protein called Protein Kinase A (PKA). PKA activation triggers a cascade that suppresses the production of pro-inflammatory cytokines, the chemical messengers that recruit more immune cells and sustain the inflammatory cycle.1PubMed. Exploring the Therapeutic Potential of Crisaborole Gel in Atopic Dermatitis: A Review PDE4 exists in at least four subtypes, all primarily expressed in immune cells, and blocking them lengthens the duration of cAMP signals broadly across those cell populations.2PubMed Central. Chemical, Biochemical, and Structural Similarities and Differences of Dermatological cAMP Phosphodiesterase-IV Inhibitors
The Role of Boron in Crisaborole’s Design
Most drugs are built around carbon, nitrogen, and oxygen. Crisaborole belongs to a class of molecules called benzoxaboroles, which incorporate a boron atom into their structure. This is not just a chemical curiosity. The boron atom is what allows crisaborole to bind tightly and selectively to PDE4’s active site. It forms a reversible bond with a specific metal ion inside the enzyme’s catalytic pocket, locking crisaborole in place and preventing PDE4 from doing its job.3PubMed. Crisaborole Topical Ointment, 2%: A Nonsteroidal, Topical, Anti-Inflammatory Phosphodiesterase 4 Inhibitor in Clinical Development for the Treatment of Atopic Dermatitis
The boron chemistry also keeps the molecule small. Crisaborole has a molecular weight of about 251 daltons, which is quite compact for a drug. That small size is a practical advantage because smaller molecules pass through the skin more easily. Topical drugs that are too large or too water-soluble tend to sit on the surface without reaching the cells they need to affect. Crisaborole’s combination of low molecular weight and favorable chemistry means it penetrates the skin barrier effectively and reaches the immune cells in the dermis where PDE4 overactivity is doing damage.
Which Inflammatory Signals Get Turned Down
The inflammatory environment in atopic dermatitis involves several overlapping immune pathways. Research using human blood cells has shown that PDE4 inhibition reduces the production of multiple cytokines at once, including TNF-α, IL-4, IL-13, IL-12/23p40, and IFN-γ.4Elsevier ScienceDirect / European Journal of Pharmacology. Anti-inflammatory effects of a novel phosphodiesterase-4 inhibitor, AA6216, in mouse dermatitis models Each of these plays a different role in keeping skin inflamed. IL-4 and IL-13 are central to the type of immune response (Th2-driven) that dominates atopic dermatitis and contributes to barrier dysfunction. TNF-α is a broad-acting inflammatory signal. IFN-γ is involved in a different immune arm (Th1) that can overlap in chronic disease.
A proteomic study looking at skin biopsies from patients with mild to moderate atopic dermatitis found that crisaborole treatment significantly reversed the abnormal protein profile in lesional skin, pushing it back toward the pattern seen in healthy, non-lesional skin. Key markers of Th2, Th17/Th22, and T-cell activation pathways were all shifted toward normal.5Elsevier / Journal of the American Academy of Dermatology. Crisaborole reverses dysregulation of the mild to moderate atopic dermatitis proteome toward nonlesional and normal skin This broad correction across multiple immune pathways, rather than targeting just one cytokine, is part of what distinguishes PDE4 inhibition from biologics that block a single molecule.
How Crisaborole Reduces Itch and Pain
Itch is often the most disruptive symptom for people with atopic dermatitis, and crisaborole appears to address it through a mechanism that goes beyond simply calming inflammation. Studies in mouse models of atopic dermatitis found that topical crisaborole reduced both itch-related and pain-related behaviors. Researchers traced this partly to the drug’s ability to prevent neutrophils, a type of white blood cell, from infiltrating the skin. These neutrophils carry proteins called S100A8 and S100A9 that are linked to itch signaling.6PubMed Central. Crisaborole Inhibits Itch and Pain by Preventing Neutrophil Infiltration in a Mouse Model of Atopic Dermatitis
A separate study using a different atopic dermatitis mouse model confirmed that crisaborole suppressed the expression of neutrophil-related cytokines and chemokines, reducing neutrophil migration into the skin.7Itch. Crisaborole prevents infiltration of neutrophils to suppress itch in a mouse model of atopic dermatitis This suggests that crisaborole’s anti-itch effect is not just a side benefit of reducing visible inflammation. It actively disrupts a specific cellular pathway that drives the itch-scratch cycle. For patients, this matters because itch often persists even when redness improves, and treatments that address it through a distinct mechanism can make a noticeable difference in quality of life.
Why It Does Not Thin the Skin Like Steroids
Topical corticosteroids have been the standard treatment for atopic dermatitis for decades, and they are effective. But one of their well-known drawbacks is skin thinning (atrophy), especially with prolonged use on sensitive areas like the face, eyelids, and skin folds. This happens because steroids broadly suppress cellular activity, including the production of collagen and other structural proteins in the skin.
Crisaborole works through a completely different mechanism. Because it targets PDE4 specifically rather than broadly suppressing immune and structural cell activity, it does not carry the same atrophy risk. Preclinical work with benzoxaborole PDE4 inhibitors showed that repeated application did not cause skin thinning in mouse models, unlike glucocorticoids.8The Journal of Pharmacology and Experimental Therapeutics. Treatment of Skin Inflammation with Benzoxaborole Phosphodiesterase Inhibitors: Selectivity, Cellular Activity, and Effect on Cytokines Associated with Skin Inflammation and Skin Architecture Changes This safety profile is one of the reasons crisaborole is positioned as a steroid-sparing option, particularly for long-term maintenance therapy or for areas where steroid use is risky.
Calcineurin inhibitors like tacrolimus and pimecrolimus are the other major class of non-steroidal topical treatments for atopic dermatitis. They work by blocking calcineurin, a different intracellular enzyme that activates T cells. Crisaborole and calcineurin inhibitors both avoid skin thinning, but their mechanisms are distinct. Calcineurin inhibitors directly prevent T-cell activation, while crisaborole raises cAMP levels to broadly dampen cytokine production across several immune cell types. In practice, some patients respond better to one than the other, and dermatologists sometimes rotate between them.
How Crisaborole Compares to Other PDE4 Inhibitors
Crisaborole is not the only PDE4 inhibitor used in dermatology. Apremilast is an oral PDE4 inhibitor approved for psoriasis and psoriatic arthritis, and roflumilast is a newer topical PDE4 inhibitor now approved for psoriasis, seborrheic dermatitis, and atopic dermatitis. All three are rationally designed to target the same enzyme family, but they differ substantially in potency, delivery route, and how deeply they penetrate the pharmacological target.
In laboratory comparisons, roflumilast is dramatically more potent than crisaborole. Its half-maximal inhibitory concentration (the amount needed to block half of PDE4 activity) is about 0.7 nanomolar, while crisaborole’s is around 240 nanomolar, a difference of more than 300-fold.9PubMed Central. Chemical, Biochemical, and Structural Similarities and Differences of Dermatological cAMP Phosphodiesterase-IV Inhibitors That does not mean roflumilast is automatically the better choice for every patient. Potency in a test tube does not translate directly into clinical superiority because absorption, formulation, tolerability, and the specific disease being treated all matter. Crisaborole was the first topical PDE4 inhibitor to reach the market for atopic dermatitis and has a longer track record of real-world use, while roflumilast is expanding the PDE4 inhibitor approach into additional skin conditions.
Apremilast, taken as a pill, works systemically. That means it reaches PDE4 throughout the body, not just in the skin, which is useful for conditions like psoriatic arthritis where inflammation is happening in joints. But systemic PDE4 inhibition can cause gastrointestinal side effects like nausea and diarrhea, which are uncommon with topical drugs like crisaborole. The topical delivery of crisaborole keeps the drug concentrated in the skin and limits the amount that enters the bloodstream.
Systemic Absorption and Safety Across Age Groups
Because crisaborole is applied to the skin, some of it inevitably reaches the bloodstream. The question is whether enough gets absorbed systemically to cause problems. Pharmacokinetic modeling has shown that the biggest predictor of how much crisaborole enters the bloodstream is whether the skin barrier is compromised. Patients with atopic dermatitis or psoriasis absorb roughly two and a half times more crisaborole than healthy volunteers applying the same amount, because inflamed skin is leakier.10PubMed Central. Predictors of Systemic Exposure to Topical Crisaborole: A Nonlinear Regression Analysis
The same modeling work found that systemic exposure was similar across age groups from two years old and up when the same percentage of body surface area was treated. This is relevant because children often have higher body-surface-area-to-weight ratios than adults, which could theoretically lead to proportionally higher drug absorption. The data indicated that even in the most extreme scenario, a pediatric patient treating up to 90% of their body surface area would not exceed the systemic exposure seen in adults.11PubMed Central. Predictors of Systemic Exposure to Topical Crisaborole: A Nonlinear Regression Analysis Crisaborole is approved for children as young as three months in the United States, making it one of the few non-steroidal options available for very young patients with mild to moderate atopic dermatitis.
Application Site Pain and Practical Considerations
The most commonly reported side effect of crisaborole is a burning or stinging sensation at the application site. This tends to happen within the first few minutes after applying the ointment and usually fades quickly. It is more common in people with more severely disrupted skin barriers and tends to lessen over time as the skin heals. The mechanism behind the stinging is not entirely clear, but it may relate to the ointment base or to crisaborole’s interaction with sensory nerve endings in damaged skin.
For patients who find the stinging intolerable, some dermatologists suggest refrigerating the ointment before application, which can dampen the sensation. Others recommend applying a thin layer of bland emollient first to buffer the contact. These are practical workarounds, not fixes for an underlying safety concern. The stinging does not indicate skin damage and is not related to the PDE4 inhibition mechanism itself.
Crisaborole is formulated as a 2% ointment and is applied twice daily to affected areas. Unlike topical steroids, there is no potency-class system or body-site restriction, and it does not require tapering. It is generally used for mild to moderate atopic dermatitis, not for severe disease, where systemic therapies or biologics are more appropriate. The drug fills a specific gap in the treatment ladder: patients who need something more effective than emollients alone but want to avoid the long-term risks of steroids or the immune suppression of calcineurin inhibitors.
Emerging Research on PDE4 Inhibition Beyond Inflammation
PDE4 inhibition does more in the skin than just suppress inflammation. Because cAMP signaling is involved in many cellular processes, researchers are investigating whether PDE4 inhibitors have effects on skin barrier repair, wound healing, and even pigmentation. Early evidence suggests that elevated cAMP can promote the expression of proteins involved in building and maintaining the skin barrier, which could help address one of the root problems in atopic dermatitis: a leaky skin barrier that allows allergens and irritants in.
There is also interest in whether PDE4 inhibitors could be useful in other inflammatory skin conditions beyond the ones already approved. The fact that crisaborole modulates multiple immune pathways rather than targeting a single cytokine makes it a candidate for conditions driven by overlapping inflammatory mechanisms. Research into PDE4 inhibition in contexts like allergic contact dermatitis, alopecia areata, and vitiligo is at various stages, though none of these uses are currently approved for crisaborole. The broader point is that the mechanism of action, raising cAMP to dampen immune overactivation, is relevant to a wide range of skin diseases, and dermatological PDE4 inhibitors are likely to expand in scope over the coming years.