A novel approach to target skin photodamage: Topical application of salt inducible kinase inhibitors

Abstract

Background

Ultraviolet (UV) radiation accelerates skin damage and photoageing, leading to visible signs such as wrinkles, loss of elasticity and uneven pigmentation. UV radiation causes direct DNA damage, primarily through the formation of cyclobutane pyrimidine dimers (CPDs) and 6–4 photoproducts (6‐4PPs), which can lead to mutations and cellular dysfunction if not repaired. While natural defence mechanisms like melanin production and DNA repair pathways mitigate this damage, prolonged or excessive UV exposure can overwhelm these defences, resulting in cumulative skin damage. The melanocortin 1 receptor (MC1R) plays a key role in melanogenesis and also appears to play a role in DNA repair. Salt‐inducible kinases (SIKs), critical enzymes in the MC1R pathway, are known to influence melanin production, but their role in DNA repair and photodamage remains unclear.

Objective

This study investigated the role of SIK in DNA repair and photodamage, focusing on two novel cosmetic ingredients, SIK inhibitors, coded SLT‐008 and SLT‐001.

Methods

The inhibitory effects of the ingredients on SIK activity were measured using biochemical and cellular assays. Their safety profiles were evaluated through in vitro studies and clinical trials. To analyse their impact on UV‐B‐induced DNA damage and repair, both inhibitors were topically applied to skin extracts in an ex vivo model. Finally, clinical studies were conducted in healthy volunteers irradiated with UV‐R. Efficacy was determined by measuring CPD levels, matrix metalloproteinase‐1 (MMP‐1), expression and erythema formation following UV exposure.

Results

Both ingredients effectively inhibited SIK activity and demonstrated good safety profiles. Ex vivo experiments revealed that immediate post‐UV‐B application of both ingredients significantly reduced UV‐B‐induced DNA damage, as shown by decreased CPDs, and promoted tissue repair. Additionally, both inhibitors suppressed MMP‐1 expression, an enzyme that plays a key role in the breakdown of collagen, thereby accelerating photoageing. These findings were confirmed in the clinical study, which demonstrated that topically applied SLT‐001 enhanced DNA repair, reduced MMP‐1 expression and decreased erythema formation.

Conclusion

Here we described the comprehensive role of SIK inhibition in DNA and dermal repair. This highlights its crucial role in protecting skin against UV‐induced photodamage and offering broad protection against actinic ageing.

UV‐induced photodamage leads to increased DNA damage and elevated MMP levels, which degrade collagen and contribute to wrinkle formation. SLT‐001 and SLT‐008, novel cosmetic ingredients that target salt‐inducible kinase, help reverse this effect by enhancing DNA repair, reducing MMP expression and decreasing erythema, thereby mitigating photodamage.

INTRODUCTION

Ultraviolet (UV) radiation is a major environmental factor that contributes to skin photodamage and photoageing [1]. Prolonged or excessive exposure to UV radiation accelerates visible signs of ageing, such as wrinkles, loss of elasticity and pigmentation irregularities, while also increasing the risk of skin cancer. At the molecular level, UV‐induced direct DNA damage manifests as the formation of cyclobutane pyrimidine dimers (CPDs) and 6–4 photoproducts (6‐4PPs), which disrupt normal DNA replication and transcription. If left unrepaired, these lesions can lead to mutations, cellular dysfunction and premature ageing. To counteract this damage, cells rely on DNA repair mechanisms such as nucleotide excision repair (NER) and base excision repair (BER), which play a critical role in maintaining genomic stability by removing UV‐induced CPDs and 6‐4PPs [2]. In addition to DNA repair, melanin (dark melanin eumelanin) production is a key strategy that mitigates UV‐induced damage by absorbing and scattering harmful radiation. Melanocytes synthesize melanin and transfer it to keratinocytes, where it forms supranuclear caps that shield DNA from UV‐induced lesions [3]. Interestingly, studies indicate that DNA repair varies between skin types. People with darker skin not only experience less UVR‐induced DNA damage due to the pigmented epidermis acting as an efficient UV filter compared to lighter skin, but they also exhibit greater efficiency in removing damaged cells, suggesting that DNA repair mechanisms are more effective [3, 4, 5]. However, persistent or excessive UV exposure can overwhelm these defence systems, leading to cumulative damage that accelerates photoageing.

The melanocortin 1 receptor (MC1R) is a critical regulator of the skin's response to UV radiation, playing a dual role in both melanin production and DNA repair. Activation of MC1R by α‐melanocyte‐stimulating hormone (α‐MSH) leads to increased intracellular cAMP levels, which drive eumelanin synthesis through the master transcription factor microphthalmia‐associated transcription factor (MITF) upregulation [6, 7]. Beyond its role in pigmentation, MC1R activation has been shown to enhance DNA repair efficiency by improving NER function in keratinocytes and melanocytes. Studies suggest that MC1R‐mediated cAMP signalling promotes the recruitment of DNA repair factors. However, the precise molecular mechanisms linking MC1R activation to DNA repair remain incompletely understood, warranting further investigation [8, 9, 10]. Salt‐inducible kinases (SIKs) play a key role in regulating MC1R‐mediated responses by controlling the activity of cAMP‐regulated transcriptional coactivators (CRTCs) [11, 12, 13]. SIK inhibition promotes CRTC translocation into the nucleus, enhancing MITF activity and increasing melanin synthesis. Given the interplay between MC1R, cAMP signalling and DNA repair, SIKs may serve as critical modulators of photoprotection, yet participation by SIKs as a broad class in DNA repair modulation remains unclear.

In addition to DNA damage, UV exposure accelerates photoageing through the degradation of the extracellular matrix (ECM), primarily mediated by matrix metalloproteinase‐1 (MMP‐1) [14, 15, 16]. MMP‐1, also known as collagenase‐1, is an enzyme responsible for breaking down type I collagen (COL‐1), a key structural component of the dermis in addition to other ECM proteins. UV‐induced oxidative stress and inflammation trigger the upregulation of MMP‐1, leading to collagen degradation, loss of skin elasticity and wrinkle formation [14, 15, 16]. The persistent activation of MMP‐1 contributes to the progressive weakening of the dermal structure, exacerbating visible signs of ageing. Inhibition of MMP‐1 expression has been proposed as a potential strategy to mitigate photoageing by preserving ECM integrity [15].

Here, we show that SIK inhibition plays a vital role in mitigating photodamage. Using novel SIK inhibitors developed in our laboratory, we demonstrate that their topical application significantly enhances DNA repair in ex vivo skin models by reducing CPD‐mediated DNA damage. Additionally, we highlight that SIK inhibitors effectively lower MMP‐1 expression, thereby protecting against ECM degradation linked to photoageing. Finally, clinical studies confirmed that the topical application of our inhibitors not only improved DNA repair efficiency and reduced MMP‐1 expression but also alleviated UV‐induced erythema in vivo. These findings suggest that these inhibitors hold considerable potential for reducing photodamage and modulating the skin ageing process by targeting key mechanisms in DNA repair and ECM preservation.

MATERIALS AND METHODS

SIK inhibitors

SLT‐008 and SLT‐001 were synthesized and fully characterized by Nuvisan France SARL (06905 Sophia Antipolis, France).

SIK inhibition potency

Inhibitory potencies of the ingredients against SIKs were determined using Eurofins KinaseProfiler™ Service Assay Protocols as described: Human SIK proteins are incubated with 8 mM MOPS, pH 7.0, 0.2 mM EDTA, 100 μM of substrate peptide AMARAASAAALARRR (for SIK1 assay) or KKKVSRSGLYRSPSMPENLNRPR (for SIK2 and SIK3 assays), 10 mM Magnesium acetate and [γ‐³³P]‐ATP (specific activity and concentration as required). The reaction is initiated by addition of the Mg/ATP mix. After incubation for 40 min at room temperature, the reaction is stopped by the addition of phosphoric acid to a concentration of 0.5%. 10 μL of the reaction is then spotted onto a P30 filtermat and washed four times for 4 min in 0.425% phosphoric acid and once in methanol prior to drying and scintillation counting (https://www.eurofinsdiscovery.com/solution/kinase‐profiler).

The NanoBRET Target Engagement Assay in HEK293 cells transiently transfected with SIK1‐NanoLuc Fusion Vector was performed by Reaction Biology. The detailed protocol is available on the Reaction Biology website: https://www.reactionbiology.com/services/kinase‐assays/nanobret‐intracellular‐kinase‐assay.

Formulations

For ex vivo experiments, the SIK inhibitors (SLT‐008, SLT‐001) and SLT‐043 were dissolved in 30% propylene glycol/70% ethanol at different concentrations ranging from 0% (placebo) to 2%. For clinical studies, the SIK inhibitor SLT‐001 was incorporated at 2% in a hydroalcoholic gel comprising 70% ethanol, 20% water, 7% butylene glycol, 1% Sepimax Zen and a hydroalcoholic gel comprising SLT‐001 2%, ethanol 70%, water 20%, butylene glycol 7%, dimethyl isosorbide 5% and Sepimax Zen 1%.

Human skin explants were obtained from surgical waste in full accordance with the declaration of Helsinki and Article L.1243–4 of the French Public Health Code. The latter does not require prior authorization by an ethics committee for the sampling and usage of surgical waste. Human skin explants with an average diameter of 11 mm (±1 mm) were kept viable in BIO‐EC's Explant Medium (BEM) at 37°C in a humid, 5% CO₂ atmosphere. Products were applied topically, 2 μL per explant and spread using a small spatula.

Before UV irradiation, the culture medium of the explants was replaced with HBSS (Hank's Balanced Saline Solution, 1 mL per explant). Then, the explants were irradiated using an RMX 3 W UV simulator (Vilber Lourmat, Marne‐la‐Vallée). UV‐B was applied at a dose of 0.3 J/cm², corresponding to 2 MED on skin with an II–III phototype. At the end of the UV irradiation, the explant medium was changed back to BEM.

Cell culture

Normal human fibroblasts (NHFs) were isolated from foreskin and cultured in 6‐well plates at a density of 1.5 × 10⁵ cells per well in 2 mL of DMEM with 10% FBS. After 24 h, cells were treated with SLT‐008 or DMSO as control.

Following an additional 24‐h incubation, NHFs were irradiated with UV‐A (BS‐02 from « Opsytec Dr. Grobel) while maintained in 1 mL PBS. Immediately after irradiation, cells were treated again with SLT‐008 or DMSO, and all conditions were incubated in 1.5 mL of fresh DMEM with 10% FBS for 24 h.

Protein extraction and western blot analysis

After 24 h, the supernatant was collected, and cells were lysed for protein analysis. Cells were resuspended in RIPA lysis buffer (Sigma‐Aldrich) containing phosphatase inhibitors (PhosSTOP, Roche) and protease inhibitors (Complete, Roche, Basel, Switzerland). Lysates were centrifuged at 14000 rpm for 10 min at 4°C, and the total protein content was determined using the Bio‐Rad protein assay. For Western blot analysis, cell lysates were separated by SDS‐PAGE, transferred onto a PVDF membrane and incubated with the appropriate primary antibodies. Protein bands were detected using ECL (enhanced chemiluminescence) and visualized with a Fujifilm LAS‐4000 system. Protein quantification was performed using MultiGauge and ImageJ software.

ELISA

Medium was collected to measure secreted MMP‐active following the protocol of Fluorokine E Kit, F1M00 (Human Active MMP‐1 ELISA; Biotechne).

Histology

Samples were fixed in buffered formalin solution for 24 h, then the samples were dehydrated and impregnated in paraffin using a Leica PEARL dehydration automat (Leica Biosystems). The samples were embedded using a Leica EG 1160 embedding station. Five‐micrometre thick sections were obtained using a Leica RM 2125 Minot‐type microtome, and the sections were mounted on Superfrost® histological glass slides. The microscopic observations were realized using a Leica DMLB, Olympus BX43 or Olympus BX63 microscope.

CPD immunostaining

CPD immunostaining was performed on formalin‐fixed, paraffin‐embedded (FFPE) skin sections with a monoclonal anti‐CPD antibody (Kamiya, MC‐062, clone KTM53) diluted 1:1600 in PBS containing 0.3% BSA and 0.05% Tween 20 and incubated for 1 h at room temperature using a VECTASTAIN® Elite® ABC‐HRP Kit (Vector Laboratories; PK‐7200) with HRP substrate VIP (Vector Laboratories, SK‐4600), which gives a violet signal once oxidized. The staining was semi‐quantified by image analysis using cellSens software (Olympus).

For the clinical study, slides underwent on‐board deparaffinization and antigen retrieval with CC1 (Ventana 950–500) standard conditions. Subsequently, the primary antibody was manually applied at a dilution of 1:2000 and incubated at 37C for 1 h, followed by an 8‐min endogenous peroxidase quench. The primary antibody was linked with Ventana anti‐mouse‐HQ (Ventana 760–4814) and anti‐HQ‐HRP (Ventana 760–4820) for 20 min each and staining was visualized with DAB (Ventana 760–159). Slides were counterstained with Mayer's haematoxylin and mounted. Assay performance was monitored by the inclusion of a positive control, known UV‐damaged skin.

Cell viability of epidermal and dermal structures was assessed by microscopic observation of FFPE skin sections after Masson's trichrome staining, Goldner variant.

In vitro Safety Testing Strategy was performed using regulatory OECD‐accepted test methods in compliance with GLP: skin irritation (TG 439) by a cytotoxicity test using the SkinEthic® Reconstructed Human Epidermis (RHE) model, phototoxicity by a photoirritation method using the 3T3NRU (Neutral Red Uptake) assay (TG 498), mutagenicity by the bacterial reverse mutation Ames test (TG 471) and genotoxicity by the micronucleus test using cultured human lymphocytes (TG 487). Due to the high lipophilicity of SLT‐008, no OECD test method was able to assess its skin sensitizing potential. To avoid false‐negative results, a weight of evidence approach was performed and allowed assessment of skin sensitization using RHE for the SENS‐IL18 and SENS‐IS test methods (EpiCS® Phenion® [Henkel] and Episkin®, respectively). The SENS‐IS assay was performed using the ECVAM DB‐ALM Invittox draft protocol (February 2009/vers. 6), in compliance with GLP, and SENS‐IL18 was performed using the protocol of Andres et al. [17]. The Genomic Allergen Rapid Detection assay evaluates the transcriptional patterns of an endpoint‐specific genomic biomarker signature in the SenzaCell® cell line to identify skin sensitizers (SenzaGen GARD®skin). GARD®skin has been recently included in the OECD TG 442E and was used for further SLT‐001 testing.

In silico predictions (DEREK NEXUS and Leadscope) and experimental results were combined to identify safe concentrations of SLT‐001 and SLT‐008 for use as cosmetic ingredients.

Clinical photodamage repair efficacy study

The studies were conducted by Product Investigations, Inc. CRO; both studies were randomized, controlled, blinded design in which the study staff and the panellists did not know the identity of the products.

Subjects

In the first study, 14 healthy females aged 18–45 years with Fitzpatrick skin phototypes I and II were enrolled and completed the study (Table 2). In the second study, 9 healthy females aged 18–45 years with skin phototypes I and II were enrolled (Table 3). The protocol was reviewed by the Sterling IRB by expedited review (determined by Sterling IRB to meet the criteria of minimal risk) and approved on 09 July 2024. The approved study documents were as follows: Protocol (Version 1.0, 20 June 2024); Participant Informed Consent Form (Version Date: 20 June 2024); written informed consent was obtained from each panellist before conducting any study‐specific procedure. The panellist was provided with a consent form that adheres to Title 21 of the Code of Federal Regulations, Part 50, Subpart B. The panellist was given ample time to read and consider the information provided and the opportunity to ask questions prior to signing the document. Panellists were informed that they are free to discontinue from the study at any time. The investigator retained the original, signed Informed Consent document and a copy was given to the panellist.

TABLE 2.

Patients DE randomized from the first clinical trial.

Subject number	Age	Fitzpatrick skin type	ITA
1	42	II	27.98
2	28	II	40.04
3	30	II	56.57
4	30	II	56.31
5	29	II	41.34
6	36	II	63.7
7	32	II	20.5
8	27	II	56.48
9	29	II	58.41
10	29	II	56.9
11	29	II	57.99
12	36	II	51.7
13	32	II	23.31
14	27	II	42.6

TABLE 3.

Patients DE randomized from the second clinical trial.

Subject number	Age	Fitzpatrick skin type	ITA
1	42	II	27.7
2	28	II	33.6
3	30	II	53.8
4	30	II	47.2
5	29	II	61.2
6	36	II	49.6
7	32	II	34.9
8	27	II	59.5
9	29	II	47.6

Exclusion criteria

Participants with chronic medical conditions, recent cancer treatment or skin disorders were excluded. Those using isotretinoin, retinoids, AHAs, immunosuppressants, steroids, anticoagulants or anti‐platelet medications (except low‐dose aspirin) were ineligible. Additional exclusions included allergies to lidocaine, epinephrine or latex, pregnancy or lactation, recent COVID‐19 exposure and study‐affiliated employees.

Biopsies

A 2‐mm punch biopsy was performed on designated skin sites following standard clinical protocols. The biopsy sites were first cleansed with 70% isopropyl alcohol, and local anaesthesia was administered using lidocaine with epinephrine to ensure participant comfort. Once adequate anaesthesia was achieved, the punch biopsy was performed and the skin sample was carefully extracted. The biopsy sites were then treated and dressed appropriately. Participants received detailed wound care instructions and were scheduled for a follow‐up assessment on Days 5–7, with additional wound care visits arranged if necessary.

UV irradiation and product application: Two template areas (22.26 cm² each) were marked on opposite sides of the thoracic back, equidistant from the midline and at the same spinal level. Test products were applied at 2 mg/cm² using a pipette to dispense 45.0 μL per area. Applications were conducted 24 h before UVA/UVB exposure (290‐400 nm), immediately after, and 12 h post‐exposure. UV irradiation was performed using the Solar Light Company Model 601–300 V2.5 Multiport, ensuring 95% uniformity across six 8 mm² exposure sites, following ISO, FDA, JCIA and COLIPA spectral irradiance guidelines. Each subject received 1 minimal erythema dose (MED), determined by exposing six lower back port sites to increasing intensities using a Solar Simulator. The intensities (262, 328, 410, 512, 640 and 800 microjoules/cm²) increased by a factor of 1.25 per port. Exposure time was identical across ports. The MED was defined as the lowest intensity required to induce grade 1 erythema at 24 h post‐exposure, calculated as intensity × exposure time.

Erythema measurement

Cortex DSM‐4 Colorimeter provides a measurement area of 4.2 mm² and values of ITA, erythema, melanin, CIE‐L*, CIE‐a* and CIE‐b*. The primary measure of erythema was Da (after exposure CIE a* value – pre‐exposure CIE a* value). For instrumental erythema data, the difference from baseline was calculated for each timepoint and a paired t‐test (or Wilcoxon signed‐rank test) was used to compare the data to baseline. To compare two treatments, the difference from baseline was calculated for each treatment and a paired t‐test (or Wilcoxon signed‐rank test) was used to compare the treatments at each time point.

Statistical analysis

All the statistical analyses were performed using GraphPad Prism software Version 10.1.1 (270). All data are expressed as mean ± SEM; *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001.

Image analysis

Images of the selected batch were analysed using either cellSens software or Image J software.

RESULTS

SLT‐008 and SLT‐001 novel SIK inhibitors

A series of SIK inhibitors featuring a pyrimidine‐pyridone pharmacophore have been developed. The incorporated substituents allow for modulation of lipophilicity and physical properties, which, in turn, influence skin penetration. SLT‐008 (Figure 1a) is a novel SIK inhibitor with high lipophilicity (clogP 6.66; Property Calculator, Molinspirations, Slovak Republic) and a molecular weight of 577.78. Similarly, SLT‐001 (Figure 1B) has a clogP value of 5.2 and a molecular weight of 527.67 [11].

FIGURE 1.

Chemical structures of SLT‐008 (a) and SLT‐001 (b).

The biochemical potencies of SLT‐008 and SLT‐001 against the three SIK isoforms 1, 2 and 3 were determined with KinaseProfiler™ assays. SLT‐008 and SLT‐001 exhibited high potency against SIK1/2 and selectivity over SIK3, with IC₅₀ of SIK1/2/3 being 5/8/>20 nM for SLT‐008 and 7/14/>20 nM for SLT‐001. To confirm their SIK cellular target engagement, a NanoBRET assay was performed. In this assay, engagement with a fluorescent tracer molecule that binds the ATP‐binding site of SIK1 in a NanoLuc‐SIK1 fusion leads to a BRET signal. A SIK1‐binding ligand displaces the tracer, leading to a diminished signal. SLT‐008 and SLT‐001 bind to SIK1 strongly with an IC₅₀ of 9.7 and 1.9 nM, respectively.

SLT‐001 and SLT‐008 reduce DNA damage

UV‐B damages DNA directly, resulting in the formation of CPDs and 6‐4PPs [18]. Previous studies have shown that the α‐MSH/cAMP pathway can protect skin from UV‐B‐induced DNA damage [8, 19, 20] therefore we tested whether inhibition of SIK by SLT‐008 and SLT‐001 can also elicit this effect.

SLT‐008, SLT‐001 or vehicle (ethanol 70%, propylene glycol 30%) were topically administered to an explant derived from a 67‐year‐old Caucasian woman (skin phototype II). The application was administered twice, a day before and after UV‐B exposure. DNA damage was then assessed 24 h after irradiation (Protocol shown in Figure 2a).

FIGURE 2.

SIK inhibition enhances DNA repair. (a) Scheme of study treatment. (b, e) Immunostaining of CPD (violet) in skin sections 24 h post UV‐B irradiation (0.3 J/cm²). (c, f) Quantification of immunostained CPD using cellSens software; n = 9 per condition. (d) SLT‐043 chemical structure. Data are expressed as mean ± SEM. Statistical analysis was performed using ordinary one‐way ANOVA with Dunnett's post‐test for pairwise comparisons. n.s, not significant, *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001.

At 24 h post‐UV‐B exposure, CPD levels were significantly decreased by treatment with SLT‐008 (60.1%, Figure 2b,c). Treatment with SLT‐001 resulted in an even greater reduction in CPD levels, reaching 93.9% (Figure 2b,c).

To confirm the specificity of SIK activity in UV protection, we tested SIK inhibitor, SLT‐043, which has weak inhibitory activity against SIKs (>500 nM for SIK 1, 2 and 3) (Figure 2d). Topical application of SLT‐043 did not affect CPD levels and resulted in persistence of DNA damage comparable to UV exposure alone (Figure 2e,f).

Photolyase is a well‐known enzyme that repairs DNA damage caused by UV. Several sunscreens and topical creams include photolyase to help prevent and treat photoageing [21, 22, 23, 24]. We evaluated the DNA repair activity of SIK inhibitors in comparison to Photolyase. Topical application of SIK inhibitors was performed on a skin explant derived from a 55‐year‐old Caucasian woman with skin phototype II, immediately after UV‐B irradiation, as described in the protocol (Figure S1a). The results indicated no significant difference between SIK inhibitors and photolyase‐treated samples. All tested compounds significantly reduced CPD levels compared to the vehicle, with SIK inhibitors showing reductions comparable to those achieved by photolyase (Figure S1b).

Additionally, tissue viability was assessed. Consistent with the CPD results, the UV‐irradiated group showed evident damage (Figure S1d), while treatment with either SIK inhibitors or photolyase significantly improved viability. Slightly greater improvement was observed in the SIK inhibitor group, as evidenced by fewer pyknotic nuclei and reduced cellular edema (Figure S1d).

These findings indicate that SIK inhibitors, such as SLT‐001 and SLT‐008, effectively reduce UV‐induced DNA and tissue damage.

SIK inhibition attenuates UV‐induced skin damage by downregulating MMP‐1 expression

Exposure of skin to UV radiation induces upregulation of MMP‐1 expression, which leads to the breakdown of collagen, contributing to premature skin ageing (photoageing) [25, 26]. The protective effects of SIK inhibition were further evaluated by measuring MMP‐1 levels. Skin explants were treated as outlined in Figure 2a. Following UV‐B exposure, MMP‐1 levels significantly increased by 139% compared to baseline (Figure 3a,b). Treatment with SLT‐008 reduced MMP‐1 levels by 52%, while SLT‐001 achieved a greater reduction of 78% (Figure 3a,b). No significant reduction was observed with SLT‐043 (Figure 3c,d), or with the photolyase treated group (Figure S1c).

FIGURE 3.

SIK inhibition decreases UV‐B‐induced MMP‐1 expression. (a, c) Immunostaining of MMP‐1 (violet) in skin sections 24 h post UV‐B irradiation (0.3 J/cm²). (b, d) Quantification of immunostained MMP‐1 using cellSens software; n = 9 per condition. Statistical analysis was performed using ordinary one‐way ANOVA with Dunnett's post‐test for pairwise comparisons to UV only (b) or using Student's t‐test (d). *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001. Data are expressed as mean ± SEM.

To further investigate the activity of SIK inhibitors, we conducted experiments using cultured fibroblasts. Fibroblasts were irradiated with UV‐A, and MMP‐1 secretion and intracellular expression were analysed. As expected, UV‐A irradiation strongly induced MMP‐1 secretion and expression (Figure S2a–c) and these effects were completely abolished by treatment with the SIK inhibitor, SLT‐008 (Figure S2a–c).

In summary, SIK inhibition protects skin from UV‐induced damage by preserving tissue viability, mitigating MMP‐1‐mediated collagen degradation and decreasing DNA damage.

SLT‐001 and SLT‐008 possess a good safety profile

Given the promising ex vivo activity of SIK inhibitors, we decided to move forward with a clinical trial. SLT‐001 and SLT‐008 have been assigned the INCI (International Nomenclature Cosmetic Ingredient) name as Cyclobutyl Xylyl Methylpiperizinylphenylamino Dihydropyrimidopyrimidinone and Adamantyl Xylyl Methylpiperizinylphenylamino Dihydropyrimidopyrimidinone, respectively. The two novel SIK inhibitors, SLT‐001 and SLT‐008 can be used as cosmetic ingredients for topical use.

Because testing cosmetic products and their ingredients on animals is banned in the EU as in many other countries, an in vitro integrated testing strategy was performed using the Weight of Evidence approach. Relevant in vitro assays were selected for a high human toxicity prediction. The results showed a good safety profile in vitro (Table 1). However, one in vitro assay, GARD®skin (OECD 442E), yielded a positive result for SLT‐001, while another assay, SENSIS (non‐OECD), was 100% negative. Although the SENSIS assay is not OECD‐approved, it demonstrates strong predictive capability. An in silico analysis predicted an EC value indicating SLT‐001 as a weak skin sensitizer. A toxicologist reviewed both the in silico predictions and the in vitro findings to calculate the margin of safety for clinical studies.

TABLE 1.

Complete SLT‐001 and SLT‐008 in vitro safety profile.

Toxicological endpoint	OECD TG	SLT‐001	SLT‐008
Skin irritation HRE	439	Non skin irritant	Non skin irritant
Phototoxicity 3 T3 NRU	498	Non phototoxic	Non phototoxic
Mutagenicity (Ames)	471	Non mutagenic	Non mutagenic
Genotoxicity (Micronucleus)	487	Non aneugen and non clastogen	Non aneugen and non clastogen
Skin sensitization SENS‐IL18	NA	/	Non sensitizer
Skin sensitization SENS‐IS	NA	Non sensitizer	Non sensitizer
Skin sensitization GARD	442E	Sensitizer	NA

Note: Skin irritation: cytotoxicity was assessed using the SkinEthic® Reconstructed Human Epidermis (RHE) model. Phototoxicity: photo‐irritation was assessed using the 3T3NRU (Neutral Red Uptake) assay. Mutagenicity: bacterial reverse mutation Ames test was used. Genotoxicity: micronucleus test using cultured human lymphocytes. Skin sensitization potential was assessed using RHE test methods (cytokine IL‐18 release measurement (SENS‐IL18 assay) and genomic SENS‐IS) and genomic GARD assays.

Abbreviation: NA, not applicable.

No adverse events were reported throughout the studies. SLT‐001 was well tolerated with no clinical signs of intolerance and reports of discomfort at any of the studies' timepoint (data on file).

Clinical efficacy: SLT‐001 enhances DNA repair in vivo

Based on the effectiveness of SIK inhibitors observed ex vivo and the good safety data, we decided to conduct a clinical study with the primary objective of determining whether topical treatment could reverse UV‐induced photodamage. In this clinical study, 2% SLT‐001 was used. The study focused on two key parameters: DNA repair and the expression of MMP‐1.

A total of 14 healthy female volunteers (Fitzpatrick Skin Type II) received topical applications of either 2% SLT‐001 or vehicle 24 h before UV‐A/UV‐B exposure, immediately after and 12 h post‐exposure (Study Design; Figure 4a and Table 2). Skin biopsies were collected immediately and 24 h after UV exposure. Immediately post‐UV radiation, we observed a significant increase in CPD levels, with a similar increase in both the vehicle‐treated and SLT‐001 treated groups, confirming that SLT‐001 does not act as a sunscreen.

FIGURE 4.

SIK inhibition enhances DNA repair in human trial. (a) Study design. (b) Immunostaining of CPD (Brown signal) in skin biopsies immediate and 24 h post UV irradiation (1MED). (c) Quantification of immunostained CPD using image‐J software; n = 14. Statistical analysis was performed using ordinary one‐way ANOVA with Dunnett's post‐test for pairwise comparisons to vehicle. *p < 0.05, ****p < 0.0001. Data are expressed as mean ± SEM.

As expected, minimal natural DNA repair was observed in the UV vehicle‐treated group after 24 h. However, DNA repair was significantly enhanced in the SLT‐001‐treated group, with a reduction of CPD by 65.2% while only a 27.0% reduction was observed in the vehicle‐treated group (Figure 4b,c).

Clinical efficacy: SLT‐001 attenuates UV‐induced MMP‐1 increase in a human trial

MMP‐1 expression was examined immediately after UV‐R and 24 h post UV‐R. As expected, immediately post UV‐R MMP‐1 was not induced at the protein level (Figure 5a,b). However, 24 h post‐UV‐R exposure, a significant induction of MMP1 was observed in the vehicle group in both the epidermis and dermis (Figure 5a indicated by black arrows), with an increase of 77%. In contrast, in the SLT‐001 treated group, there was no significant increase (Figure 5b).

FIGURE 5.

In human trial, decrease of UV‐R induced MMP‐1 expression with SLT‐001 treatment. (a) Immunostaining of MMP‐1 (brown signal, black arrows) in skin biopsies immediate and 24 h post UV irradiation (1MED). (b) Quantification of immunostained MMP‐1 using image‐J software; n = 14. Statistical analysis was performed using ordinary one‐way ANOVA with Dunnett's post‐test for pairwise comparisons to vehicle. *p < 0.05. Data are expressed as mean ± SEM.

In conclusion, the clinical evidence demonstrates that 2% SLT‐001 significantly enhances DNA repair following UV exposure and substantially inhibits UV‐induced MMP‐1 expression.

Clinical efficacy: SLT‐001 reduces erythema formation

Exposure of human skin to UV causes erythema (redness) and DNA damage [27, 28]. Here, we evaluated erythema formation 24 h post‐UV exposure. Our findings demonstrated that the SLT‐001 treatment group showed significantly reduced erythema levels compared to the vehicle group (Figure 6a,b).

FIGURE 6.

Decrease in erythema formation with SLT‐001 treatment in two independent clinical trials. (a, c) Images of erythema formation 24 h. post UV‐R (b, d). Quantification of erythema using DSM colorimeter, statistical analysis was performed using Wilcoxon signed‐rank test assessing treatment comparisons regarding the change from baseline *p < 0.05, **p < 0.01. n = 14 (b), n = 9 (d).

To further validate if SLT‐001 can reduce erythema formation, we conducted a follow‐up clinical study (Figure 6c,d) with a similar design (as outlined in Figure 4a). Briefly, 9 healthy female subjects (Fitzpatrick skin type II) (Table 3) received topical applications of either 2% SLT‐001 or vehicle 24 h before UV‐A/UV‐B exposure, immediately after and 12 h post‐exposure. Erythema was evaluated 24 h post UV‐R. The findings confirmed that SLT‐001 treatment reproducibly reduced erythema formation (Figure 6c,d), highlighting its efficacy in mitigating the effects of UV‐R‐induced skin damage.

DISCUSSION

Salt‐inducible kinases (SIKs) play a pivotal role in melanin synthesis, as previous studies have shown that inhibition of SIK through genetic models or by small molecules enhances melanin production [11, 12]. Here we explored the effects of two novel SIK inhibitors, SLT‐001 and SLT‐008, on UV‐induced photodamage in human skin. Our data suggest that SIK inhibitors have no SPF action (at this short time period) (Figure 4c) and support the additional protection by enhancing DNA repair.

Although SIK activity in melanocytes is well‐documented, we observed that SLT‐001 and SLT‐008 also enhance DNA repair in keratinocytes and fibroblasts, as demonstrated by extensive epidermal staining (Figure 4b). This finding aligns with previous reports that melanocortin receptor 1 (MC1R), a key regulator of pigmentation, is expressed not only in melanocytes but also in keratinocytes, fibroblasts and endothelial cells [29, 30]. MC1R activation has been associated with several protective effects, including antioxidant activity, DNA repair mechanisms and immunomodulation via interleukin‐10 (IL‐10) [9]. Our data suggest that SIK inhibition may similarly confer protection across multiple skin cell types.

Our findings suggest that SIK inhibition plays a central role in mediating the cAMP‐induced enhancement of DNA repair, a mechanism not previously described. We demonstrate that SIK inhibition alone—independent of PKA activation—enhances DNA repair activity. Treatment with two chemically differentiated SIK inhibitors, SLT‐001 and SLT‐008, led to the activation of DNA repair pathways. In contrast, SLT‐043, a related compound with minimal SIK inhibitory activity, had no effect, supporting the specificity of the response. These results indicate that the observed enhancement in DNA repair is driven by SIK inhibition. While our data support the involvement of SIK in this process, the precise downstream effectors and potential interactions with components of the nucleotide excision repair (NER) pathway remain to be elucidated. Further mechanistic studies are warranted to define the exact role of SIK in regulating DNA repair and promoting skin protection.

Interestingly, we observed reduced erythema formation by SLT‐001 treated skin following UV exposure (Figure 6). While erythema has been proposed as a noninvasive surrogate for assessing UV‐induced DNA damage, its correlation with actual DNA damage remains inconsistent across studies [2, 11, 31, 32]. The reduction in erythema observed with SIK inhibitors could be attributed to enhanced DNA repair and/or anti‐inflammatory effects. SIK inhibition, similar to MC1R function, has been shown to upregulate IL‐10 production [33], which suppresses proinflammatory cytokines and modulates immune responses to UV‐induced skin damage. This dual mechanism may help explain the observed reduction in erythema intensity.

In addition to enhancing DNA repair, SLT‐001and SLT‐008 also decreased MMP‐1 expression, a key marker of photoageing. MMP‐1 is primarily secreted by fibroblasts [25] but can also be influenced by cross‐talk with keratinocytes [34]. While our study focused on UV‐B‐induced damage, we demonstrated that SIK inhibitors directly reduce UV‐A‐induced MMP‐1 expression in fibroblasts (Figure S2), though further research is needed to evaluate the efficacy of SIK inhibitors against UV‐A‐dominant solar radiation. These dual benefits—enhanced DNA repair and reduced MMP‐1 expression—position SIK inhibitors as promising agents for combating photoageing.

Recent advances in the understanding of the molecular mechanisms of photodamage and DNA repair have led to the development of innovative photoprotective strategies. These include the incorporation of DNA repair enzymes, such as photolyase and T4 endonuclease V, into sunscreens to enhance the skin's natural repair processes [24] Indeed, we have shown that our inhibitors exhibit comparable activity to photolyase in terms of DNA repair. However, our inhibitors also affect MMP‐1 regulation, suggesting multiple protection through DNA repair and the prevention of collagen degradation, both hallmarks of photoageing. Additionally, a significant advantage of our inhibitors is that, unlike photolyase, they do not require light for activation, which is counterintuitive for a UV protection claim. As photolyase‐dependent DNA repair is limited by light availability, its efficacy is constrained. In contrast, our inhibitors are active regardless of light exposure, making them suitable for concomitant use of sunscreens and day creams.

Targeting the cAMP/α‐MSH signalling pathway has been a promising strategy for skin protection. For example, afamelanotide, a synthetic α‐MSH analogue, enhances both pigmentation and DNA repair [10, 35]. However, afamelanotide requires intradermal delivery and is associated with significant side effects. Similarly, topical agents such as forskolin have shown protective effects in preclinical models but face challenges in clinical translation possibly due to differences in skin permeability [8, 36]. In contrast, SLT‐001 and SLT‐008 were specifically designed for topical application with optimized molecular properties for skin penetration.

In conclusion, our findings suggest that topical formulations containing 1–2% SLT‐008 SLT‐001 applied both pre‐ and post‐sun exposure could offer significant protection against photodamage by enhancing DNA repair mechanisms, reducing erythema and decreasing MMP‐1 expression. The effectiveness of the DNA repair process varies across different skin types, and SIK inhibitors can effectively complement sunscreen protection. By promoting DNA repair and preventing matrix degradation, SIK inhibitors provide additional defence during and after sun exposure.

In summary, we provide evidence that SIK inhibitors, SLT‐001 and SLT‐008, offer protection against UV‐induced photodamage. Topical application of these cosmetic ingredients represents a powerful approach to mitigate UV‐induced damage and photoageing, while potentially delivering broader cosmetic benefits.

Study limitations

One limitation of this study is that all participants were females with Fitzpatrick skin type II. DNA repair kinetics are known to vary between skin types, with lighter skin types (such as type I and II) generally showing slower repair compared to darker types (III and IV) [3, 4, 5]. Therefore, the findings may not be generalized to include individuals with different skin phototypes, especially those with faster DNA repair responses. Additionally, the duration of the study was relatively short. While previous research indicates that the majority of DNA damage repair occurs within the first week, longer term studies are needed to fully understand sustained repair dynamics. Finally, the sample size was small. However, despite the limited number of participants, the study still demonstrated statistically significant effects. Future research should include a more diverse range of skin types and longer observation periods to validate and expand on these findings.

CONFLICT OF INTEREST STATEMENT

All authors have a financial interest in Soltégo, Inc. D.E.F's financial interests in Soltégo, Inc. were reviewed and are managed by Massachusetts General Hospital and Partners HealthCare in accordance with their conflict of interest policies. D.E.F. also discloses ownership and consulting relationships with Tasca, Swiss Rockets, Coherent Medicines, AME Therapeutics and Biocoz, and a consulting relationship with Pierre Fabre. BS is a consultant to Palvella Therapeutics and Ligand Pharmaceuticals.TP has received honoraria and/or consulting fees from ACM, Almirall, AbbVie, Amgen, Astellas, Beiersdorf, Bristol Myers Squibb, Caudalie, Celgene, Galderma, GlaxoSmithKline, Hyphen, Incyte, ISDIN, ISIS Pharma, Janssen, La Roche Posay, LEO Pharma, Lilly, L'Oréal, Merck Sharpe & Dohme, NAOS, Novartis, Pierre Fabre, Pfizer, Sanofi‐Genzyme, Soltego, Sun Pharmaceutical Industries, SVR, Symrise, Takeda, UCB, Vichy and VYNE Therapeutics. He is the cofounder of Nikaia Pharmaceuticals.

Supporting information

Figure S1.

Figure S2.

Rachmin I, Le Varlet B, Regazzetti C, Passeron T, Wang J, Fisher DE, et al. A novel approach to target skin photodamage: Topical application of salt inducible kinase inhibitors. Int J Cosmet Sci. 2026;48:1–15. 10.1111/ics.70003

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.