Phase II trial of SM-88, a cancer metabolism based therapy, in non-metastatic biochemical recurrent prostate cancer.

Background Androgen deprivation therapy (ADT) is a standard treatment for high-risk biochemically-recurrent, non-metastatic prostate cancer (BRPC) but is not curative and associated with toxicity. Racemetyrosine (SM-88) is an amino-acid analogue used with methoxsalen, phenytoin, and sirolimus (MPS) to enhance SM-88 activity. Method A phase 1b/2, open-label trial in BRPC and rising PSA. Patients were given daily SM-88 (230 mg BID), methoxsalen (10 mg), phenytoin (50 mg), and sirolimus (0.5 mg)). Outcome measures included changes in PSA, circulating tumor cells (CTCs) and imaging. Results 34 subjects were screened, 23 treated and 21 remained on study for ≥12 weeks. The median PSA was 6.4 ng/ml (range 1.7-80.1); doubling-time 6.2 months (range 1.4-36.6) and baseline testosterone 319.1 ng/ml (range 2.5-913.7). Median duration of therapy was 6.5 months (2.6-14.0). CTCs (median 48.5 cells/4 ml (range 15-268) at baseline) decreased a median of 65.3% in 18 of 19 patients. For patients who achieved an absolute CTC nadir count of <10 cells/4 ml (n = 10), disease control was 100% i.e. no metastases or PSA progression, while on trial (p = 0.005). PSA fell by ≥50% in 4.3% (1 subject). No patients developed metastatic disease while on treatment (metastases free survival =100%). There were no treatment-related adverse events (AEs) and quality of life was unchanged from baseline on the EORTC QLQ-C30 and QLQ-PR25. Testosterone levels rose slightly on SM-88 and were unrelated to efficacy or toxicity. Conclusions Use of SM-88 was associated with disease control while maintaining QOL. SM-88 may delay the need for ADT and the associated hormonal side effects. Larger trials are planned.Trial registration number, date of registration - NCT02796898, June 13, 2016.

Xanthotoxin affects depression-related behavior and neurotransmitters content in a sex-dependent manner in mice.

The present study aimed to evaluate xanthotoxin's influence on male and female Swiss mice's depression-like behaviors and investigate the potential mechanism of this effect. Naturally derived furanocoumarin (the Apiaceae family), xanthotoxin, administered acutely (12.5 mg/kg), diminished the immobility level in the forced swim test only in males. The immobility level was lower in females than males, which may be associated with a higher serotonin level in the female prefrontal cortex. A dose-dependent increase of serotonin and noradrenaline was reported in the reverse-phase ion-pair liquid chromatography in the female prefrontal cortex but not in the hippocampus. We suggest that xanthotoxin may exert antidepressant properties and affect males and females differently. The increasing level of serotonin in the male and female prefrontal cortex may underlie this effect.

Novel inactivation of the causative fungal pathogen of white-nose syndrome with methoxsalen plus ultraviolet A or B radiation.

White-nose syndrome is a fungal disease responsible for the rapid decline of North American bat populations. This study addressed a novel method for inactivating Pseudogymnoascus destructans, the causative agent of WNS, using ultraviolet A (UVA) or B (UVB) radiation in combination with methoxsalen, a photosensitizer from the furanocoumarin family of compounds. Fungal spore suspensions were diluted in micromolar concentrations of methoxsalen (50-500 µM), then exposed to fixed doses of UVA radiation (500-5000 mJ/cm2), followed by plating on germination media. These plates were examined for two to four weeks for evidence of spore germination or inactivation, along with resultant growth or inhibition of P. destructans colonies. Pretreatment of fungal spores with low doses of methoxsalen resulted in a UVA dose-dependent inactivation of the P. destructans spores. All doses of methoxsalen paired with 500 mJ/cm2 of UVA led to an approximate two-log10 (~99%) reduction in spore viability, and when paired with 1000 mJ/cm2, a four-log10 or greater (>99.99%) reduction in spore viability was observed. Additionally, actively growing P. destructans colonies treated directly with methoxsalen and either UVA or UVB radiation demonstrated UV dose-dependent inhibition and termination of colony growth. This novel approach of using a photosensitizer in combination with UV radiation to control fungal growth may have broad, practical application in the future.

Randomized controlled study of ECP with methoxsalen as first-line treatment of patients with moderate to severe cGVHD.

The investigation of extracorporeal photopheresis (ECP) plus standard of care (SoC) (SoC+ECP) in chronic graft-versus-host disease (cGVHD) within prospective, randomized clinical studies is limited, despite its frequent clinical use. This phase 1/pilot study was the first randomized, prospective study to investigate ECP use as first-line therapy in cGVHD, based on the 2015 National Institutes of Health (NIH) consensus criteria for diagnosis and response assessment. Adult patients with new-onset (≤3 years of hematopoietic stem cell transplantation) moderate or severe cGVHD were randomized 1:1 to 26 weeks of SoC+ECP vs SoC (corticosteroids and cyclosporine A/tacrolimus) between 2011 and 2015. The primary endpoint was overall response rate (ORR), defined as complete or partial response, at week 28 in the intention-to-treat population (ITT). Other outcomes included quality of life (QoL) measures and safety. Sixty patients were randomized; ITT included 53 patients (SoC+ECP: 29; SoC: 24). Week 28 ORR was 74.1% (SoC+ECP) and 60.9% (SoC). Investigator-assessed ORR was 56.0% (SoC+ECP) and 66.7% (SoC). Patients treated with SoC experienced a decline in QoL over the 28-week study period; QoL remained unchanged in SoC+ECP patients. Most frequent treatment-emergent adverse events (TEAEs) in SoC+ECP patients were hypertension (31.0%), cough (20.7%), dyspnea (17.2%), and fatigue (17.2%). Seventeen patients (SoC+ECP: 8; SoC: 9) experienced 35 serious adverse events (SAEs). No TEAEs or SAEs were considered related to the ECP instrument or methoxsalen. The encouraging short-term results of this study could inform the design of subsequent studies. This trial was registered at www.clinicaltrials.gov as #NCT01380535.

Microemulsions vs chitosan derivative-coated microemulsions for dermal delivery of 8-methoxypsoralen.

BACKGROUND: 8-methoxypsoralen (8-MOP) is one of the most commonly utilized drugs in psoralen-ultraviolet A therapy for treatment of vitiligo. However, poor skin retention and systemic side effects limit the clinical application of 8-MOP. METHODS: Microemulsions (MEs) and chitosan derivative-coated 8-MOP MEs were developed and compared for dermal delivery of 8-MOP. Ex vivo skin retention/permeation study was performed to select the ME formulation with the highest retention:permeation ratio. Four different chitosan-coated MEs were prepared and compared with the ME formulation for their ability to distribute 8-MOP in the skin. RESULTS: Among various ME formulations developed, a formulation containing 2.9% ethyl oleate, 17.2% Cromophor EL35, 8.6% ethanol and 71.3% water showed the highest ex vivo skin retention:permeation ratio (1.98). Of four chitosan-coated MEs prepared, carboxymethyl chitosan-coated MEs (CC-MEs) and hydroxypropyl chitosan-coated MEs (HC-MEs) showed higher ex vivo skin retention:permeation ratio (1.46 and 1.84). and were selected for in vivo pharmacokinetic study. AUCskin (0-12 h) for 8-MOP MEs (4578.56 h·ng·mL-1) was higher than HC-MEs (3422.47 h·ng·mL-1), CC-MEs (2808.51 h·ng·mL-1) and tincture (1500.16 h·ng·mL-1). Also, AUCplasma (0-12 h) for MEs (39.35±13.90 h·ng·mL-1) was significantly lower than HC-MEs (66.32 h·ng·mL-1), CC-MEs (59.70 h·ng·mL-1) and tincture (73.02 h·ng·mL-1). CONCLUSION: These combined results suggested that the MEs developed could be a promising and safe alternative for targeted skin delivery of 8-MOP.

Methoxsalen and Bergapten Prevent Diabetes-Induced Osteoporosis by the Suppression of Osteoclastogenic Gene Expression in Mice.

This study evaluated whether bergapten and methoxsalen could prevent diabetes-induced osteoporosis and its underlying mechanism. For 10 weeks, bergapten or methoxsalen (0.02%, w/w) was applied to diabetic mice that were provided with a high-fat diet and streptozotocin. Bone mineral density (BMD) and microarchitecture quality were significantly reduced in the diabetic control group; however, both bergapten and methoxsalen reversed serum osteocalcin, bone-alkaline phosphatase and femur BMD. These coumarin derivatives significantly increased bone volume density and trabecular number, whereas they decreased the structure model index of femur tissue in diabetic mice. Conversely, tartrate-resistant acid phosphatase 5 (TRAP) staining revealed that these derivatives reduced osteoclast numbers and formation in diabetic bone tissue. Additionally, both bergapten and methoxsalen tended to downregulate the expression of osteoclast-related genes such as receptor activator of nuclear factor kappa-B ligand (RANKL), nuclear of activated T-cells, cytoplasmic 1 (NFATc1) and TRAP in diabetic femurs, with NFATc1 and TRAP expression showing significant reductions. Our data suggest that both bergapten and methoxsalen prevent diabetic osteoporosis by suppressing bone resorption.

A standardized methodical approach to characterize the influence of key parameters on the in vitro efficacy of extracorporeal photopheresis.

Extracorporeal photopheresis (ECP) is an autologous immunomodulatory cell therapy that consists of the ex vivo collection of mononuclear cells (MNCs), which are irradiated with UVA in the presence of the photosensitizing agent 8-methoxypsoralen (8-MOP) to induce cell apoptosis. This photoactivated cell preparation is then reinfused into the patient. While the clinical benefits of ECP are well-demonstrated, no study has yet characterized the influence of variations in the composition of the cell preparation on the efficacy of ECP in vitro. Here, we describe a standardized methodology for the in vitro assessment of ECP that uses the human lymphoma T-cell line and mimics the clinical procedure. By quantifying cell apoptosis, inhibition of cell proliferation, and 8-MOP consumption, we used this approach to characterize the specific influence of key variables on the cellular response to ECP. We found that (i) increases in hematocrit and plasma concentrations attenuated the cellular response to ECP; (ii) plasma concentration was the only variable tested that influenced 8-MOP consumption; and (iii) the loss of efficacy due to variations in the concentration of certain blood components could be counteracted by modulating the UVA dose. This methodology may enable evaluation of other leukapheresis preparation protocols and better determination of the optimal working parameters for ECP.

Photochemical and Pharmacokinetic Characterization of Orally Administered Chemicals to Evaluate Phototoxic Risk.

This study aimed to verify the applicability of a proposed photosafety screening system based on a reactive oxygen species (ROS) assay and a cassette-dosing pharmacokinetic (PK) study to chemicals with wide structural diversity. The orally taken chemicals, erythromycin, gatifloxacin, 8-methoxypsoralen (MOP), pirfenidone (PFD), trifluoperazine (TFP), and voriconazole (VRZ), were selected as test compounds. The ROS assay was conducted to evaluate their photoreactivity, and all test compounds excluding erythromycin generated significant ROS under simulated sunlight exposure. According to the ROS data, TFP had potent photoreactivity, and the photoreactivity of 4 other compounds was judged to be moderate. Regarding the oral cassette-dosing PK test in rats, the skin deposition of MOP, PFD, and VRZ was relatively high, and gatifloxacin and TFP exhibited moderate skin deposition properties. Based on the ROS and PK data of test compounds, PFD and TFP were judged to be potent phototoxic compounds, and MOP and VRZ were deduced to have phototoxic risk. The predicted phototoxic risk of test compounds by proposed screening was mostly in agreement with observed in vivo phototoxicity in the rat skin. The proposed screening system could provide reliable photosafety information on orally administered compounds with wide structural diversity.

Formulation characterization and in vitro drug release of hydrogel-thickened nanoemulsions for topical delivery of 8-methoxypsoralen.

Psoralens are lipophilic molecules used to treat skin diseases such as vitiligo and psoriasis, among them, 8-Methoxypsoralen (8-MOP) is the most used to topical skin application. Topical treatment with psoralens can be limited due to insufficient drug penetration into the skin layers. Nanoemulsions have attracted much attention due to their as dermal delivery systems for lipophilic drugs such as 8-MOP. However, nanoemulsions feature low viscosity, which might be unsuitable for topical application. In this work, we produced hydrogel-thickened nanoemulsions using chitosan with different molecular weight as thickening polymer to overcome the low viscosity attributed to nanoemulsions. The aim of this study is to characterize oil-in-water (o/w) nanoemulsions and hydrogel-thickened nanoemulsions based on two different essential oils to in vitro controlled release of 8-MOP. In a previous work, we have reported production, stability and in vitro transdermal release of such formulations. Here, 8-MOP loaded nanoemulsions and hydrogel-thickened nanoemulsions were characterized regarding their morphology, rheological behavior, and in vitro drug release kinetics. Hydrogel-based systems presented a gel-like rheological behavior, being classified as weak gels. 8-MOP in vitro release from hydrogels-based formulations using clove essential oil showed strong dependency chitosan molecular weight. On the other hand, hydrogel-thickened nanoemulsions using sweet fennel oil as inner phase, showed an unexpected pH-dependent behavior not fully understood at the moment. These results need further investigation, nevertheless hydrogel-thickened nanoemulsions revealed to be interesting and complex dermal delivery systems for poorly soluble drugs.

Protective effects of 8-MOP on blood-brain barrier via the Nrf-2/HO-1 pathway in mice model of cerebral infarction.

OBJECTIVE: To explore the effect of 8-MOP on the blood-brain barrier in mice model of cerebral infarction and the underlying mechanism. MATERIALS AND METHODS: Middle cerebral artery occlusion (MCAO) model was established to induce permanent cerebral infarction. The neurological function was observed and scored by the modified longa score method after model establishment. Besides, the water content of brain tissue was measured by the standard dry weight method. Evans blue exudation rate was used to evaluate the effect of 8-MOP on the permeability of the blood-brain barrier. Western-blot and quantitative polymerase chain reaction (qPCR) were used to detect the expression of MMP-9, claudin-5, vascular endothelial growth factor (VEGF), as well as the NFE2-related factor 2 (Nrf-2)/hemeoxygenase 1 (HO-1) pathway. RESULTS: 8-MOP could reduce the neurological deficit scores in a dose-dependent manner, thereby reducing cerebral edema. After 8-MOP treatment, the expression of MMP-9 decreased in ischemic brain tissue, whereas the expression of claudin-5, VEGF, and GFAP increased, suggesting that the blood-brain barrier ultrastructure was improved. In addition, the expression of Nrf-2 and HO-1 decreased after the model establishment of cerebral infarction. However, the expression of Nrf-2 and HO-1 increased in ischemic brain tissue after 8-MOP treatment. CONCLUSIONS: 8-MOP may protect the blood-brain barrier via the Nrf-2/HO-1 pathway.

Nanoemulsion containing 8-methoxypsoralen for topical treatment of dermatoses: Development, characterization and ex vivo permeation in porcine skin.

Oral therapy with 8-methoxypsoralen (8-MOP) may cause major side effects, whereas the topical treatment might not be much effective due to the low penetration induced by typical formulations. Therefore, the objectives of this work are the development and characterization of a nanoemulsion (NE) containing 8-MOP together with an ex vivo permeation study, monitored by a validated HPLC-Fluo method, to determine the amount of drug retained in viable skin (epidermis (E) and dermis (D)) and in stratum corneum (SC). The optimized conditions for NE formulation were achieved by full factorial designs (25 and 32): 60 s and 60% of ultrasound time and potency, respectively; 10 mL of final volume; 2% v/v of oil phase (clove essential oil); and 10% m/v of Poloxamer 407. The NE showed mean droplet diameter of 24.98 ±â€¯0.49 nm, polydispersity index (PDI) of 0.091 ±â€¯0.23, pH values of 6.54 ±â€¯0.06, refractive index of 1.3525 ±â€¯0.0001 and apparent viscosity of 51.15 ±â€¯3.66 mPa at 20 °C. Droplets with nanospherical diameters were also observed by transmission electron microscopy (TEM). Ex vivo permeation study showed that 8.5% of the applied 8-MOP dose permeated through the biological membranes, with flux (J) of 1.35 µg cm-2 h-1. The drug retention in E + D and in SC was 10.15 ±â€¯1.36 and 1.95 ±â€¯0.71 µg cm-2, respectively. Retention in viable skin induced by the NE was almost two-fold higher than a compounded cream (5.04 ±â€¯0.30 µg cm-2). These results suggested that the developed NE is a promising alternative for 8-MOP topical therapy when compared to commercial formulations.

Phototherapy of mycosis fungoides.

Mycosis fungoides (MF), the most common variant among cutaneous T cell lymphomas (CTCL), is characterized in its early stages by clonal proliferation of malignant T-cells in the skin manifesting as erythematous patches and plaques with a chronic course and progression to cutaneous tumors and extracutaneous organs in some patients. Skin directed therapies (SDT) are primarily used for effective palliation in early stage disease. Phototherapy with ultraviolet A radiation combined with 8-methoxypsoralen (PUVA) and with ultraviolet B radiation (UVB) has a longstanding history in the treatment of MF and are highly effective in inducing remissions. Patients with erythroderma and blood involvement benefit from treatment with extracorporeal photochemotherapy (ECP) where peripheral blood is exposed to PUVA. Phototherapy can be safely combined with systemic agents, most notably interferon-alpha and retinoids. Recently updated treatment guidelines have been published to provide evidence based algorithms for the stage-oriented treatment of MF. PUVA and narrow-band UVB (NB-UVB) are recommended as first line treatment for early stages with combination modalities reserved for refractory and more advanced cases and ECP is among the standard treatments for MF erythroderma. Areas of uncertainty relate to optimized treatment dose and schedules, the use of phototherapy for maintenance, and the role of newer phototherapeutic modalities (e.g. ultraviolet A1 radiation, excimer sources, photodynamic therapy) in the treatment of MF.

8-Methoxypsoralen disrupts MDR3-mediated phospholipids efflux and bile acid homeostasis and its relevance to hepatotoxicity.

Since its discovery in 1987, multidrug resistance 3 P-glycoprotein (MDR3) had recognized to play a crucial role in the translocation of phospholipids from the inner to outer leaflets of bile canalicular membranes. An increasing number of reports suggest that drug-mediated functional disruption of MDR3 is responsible for drug-induced cholestasis. 8-Methoxypsoralen (8-MOP) is used clinically to treat psoriasis, vitiligo and other skin disorders. However, psoralens safety for long-term use is a concern. In the current study, we evaluate 8-MOP's potential hepatotoxicity and effects on bile formation. Sprague Dawley (SD) rats were treated daily 200mg/kg or 400mg/kg of 8-MOP orally for 28 days. The result showed a prominent decrease in biliary phospholipids output, which associated with the down-regulation of MDR3. Elevated bile acid serum level and increased biliary bile acid outputs were observed in 8-MOP-treated groups. The disturbance of bile acid homeostasis was associated with changes in enzymes and proteins involved in bile acid synthesis, regulation and transport. Human liver cell line L02 was used to determine on the mRNA and protein levels of MDR3. Cells treated with 8-MOP reveled a decrease in fluorescent PC (phosphatidylcholine) secretion into the pseudocanaliculi (formed between adjacent cells) compared with untreated cells. Our investigation represent the first evidence that 8-MOP can induce cholestatic liver injury by disturbing MDR3-mediated phospholipids efflux and bile acid homeostasis.

UVA radiation augments cytotoxic activity of psoralens in melanoma cells.

PURPOSE: Melanoma is an aggressive form of skin cancer. The aim of the study was to evaluate the influence of UVA radiation and psoralens: 5-methoxypsoralen (5-MOP) or 8-methoxypsoralen (8-MOP) on melanoma cells viability. MATERIALS AND METHODS: The amelanotic C32 and melanotic COLO829 human melanoma cell lines were exposed to increasing concentrations of psoralens (0.1-100 µM) in the presence or absence of UVA radiation. Cell viability was evaluated by the WST-1 assay. RESULTS: We demonstrated that 8-MOP, in contrast to 5-MOP, has no cytotoxic effect on both melanoma cell lines. Simultaneous exposure of cells to 8-MOP and UVA radiation caused significant cytotoxic response in C32 cells where the EC50 value was estimated to be 131.0 µM (UVA dose: 1.3 J/cm2) and 105.3 µM (UVA dose: 2.6 J/cm2). The cytotoxicity of 5-MOP on both C32 and COLO829 cells was significantly augmented by UVA radiation - the EC50 was estimated to be 22.7 or 7.9 µM (UVA dose: 1.3 J/cm2) and 24.2 or 7.0 µM (UVA dose: 2.6 J/cm2), respectively. CONCLUSIONS: The demonstrated high cytotoxic response after simultaneous exposure of melanoma cells to psoralens and UVA radiation in vitro suggests the usefulness of PUVA therapy to treat melanoma in vivo.

Bath-PUVA therapy improves impaired resting regulatory T cells and increases activated regulatory T cells in psoriasis.

BACKGROUND: Bath-psoralen plus ultraviolet light A (PUVA) therapy is an effective, safe, and inexpensive treatment for psoriasis. Psoriasis might be due to an unbalanced ratio of Th17 cells and regulatory T cells (Treg). The Treg functional ratio is significantly lower in patients with psoriasis compared with controls and is inversely correlated with the Psoriasis Area and Severity Index score. We previously reported that bath-PUVA therapy significantly increases the number of Treg and restores Treg function to almost normal in most patients with psoriasis. OBJECTIVES: We examined the effects of bath-PUVA therapy on three distinct Foxp3+ subsets: activated Treg (aTreg), resting Treg (rTreg), and cytokine-secreting non-suppressive T cells. METHODS: We enrolled 15 patients with psoriasis and 11 healthy controls. We examined aTreg, rTreg, and cytokine-secreting non-suppressive T cells in peripheral blood obtained from the psoriasis patients before and after every fifth bath-PUVA therapy session. RESULTS: Levels of aTreg, which are considered to have the strongest suppressive activity in patients with psoriasis, were significantly increased in the early bath-PUVA therapy sessions, and then diminished. Levels of rTreg were lower in psoriasis patients than in healthy controls, and increased during bath-PUVA therapy. CONCLUSIONS: Bath-PUVA therapy induced aTreg and rTreg concomitantly with an improvement in the psoriatic lesions, suggesting a mechanism for the effectiveness of bath-PUVA therapy for psoriasis patients.

Enhancement of 8-methoxypsoralen topical delivery via nanosized niosomal vesicles: Formulation development, in vitro and in vivo evaluation of skin deposition.

The aim of the present study is to enhance the skin penetration and deposition of 8-methoxypsoraln (8-MOP) via niosomal vesicles to increase its local efficacy and safety. 8-MOP niosomes were prepared by the thin film hydration method using Span 60 or Span 40 along with cholesterol at five different molar ratios. The obtained vesicles revealed high entrapment efficiencies (83.04-89.90%) with nanometric vesicle diameters (111.1-198.8nm) of monodisperse distribution (PDI=0.145-0.216), zeta potential values <-48.3mV and spherical morphology under transmission electron microscopy. Optimized niosomal formulations depicted a biphasic in vitro release pattern in phosphate buffer (pH 5.5)/ethanol (7:3v/v) and displayed good physical stability after storage for 6 months at room (20-25°C) and refrigeration (4-8°C) temperatures. The two optimized formulations were incorporated in 5% sodium carboxy methylcellulose based hydrogel matrix which showed optimum pH values (7.37-7.39), pseudoplastic with thixotropic rheological behavior and more retarded 8-MOP release, by 23.82 and 14.89%, compared to niosomal vesicles after 24h. In vitro drug permeation and deposition studies, using rat skins, revealed promoted penetration and accumulation of 8-MOP after 8h. The skin penetration was further confirmed in vivo by confocal laser scanning microscopy, after 2h application period using rhodamine-loaded niosomal hydrogels compared to plain rhodamine hydrogel, as a florescence marker. Therefore, enhanced permeation and skin deposition of 8-MOP delivered by niosomes may help in improving the efficacy and safety of long-term treatment with 8-MOP.

Erythema action spectrum of topical 8-methoxypsoralen-sensitized skin re-evaluated: implications for routine clinical practice.

BACKGROUND: Published methodology used to determine psoralen plus ultraviolet A (PUVA) erythemal action spectrum does not reflect current clinical practice for psoralen sensitization. We re-evaluated the PUVA action spectrum using aqueous 8-methoxypsoralen (8-MOP) 2·6 mg L(-1) as used routinely in current clinical practice. OBJECTIVES: To determine the UVA erythema action spectrum of topical 8-MOP-sensitized normal skin. METHODS: Twenty healthy volunteers with skin phototypes I-V were recruited. Forearms were psoralen-sensitized at 37 °C for 10 min. Six UVA irradiations at 10-nm intervals between 325 and 375 nm were randomly allocated to forearm sites and were applied using a 10-nm bandwidth irradiation monochromator. The visual minimal phototoxic dose (MPD) was recorded on each site at 96 h. RESULTS: Volunteer Boston phototypes were: I, n = 2; II, n = 6; III, n = 6; IV, n = 5 and V, n = 1. The mean MPD (J cm(-2) ) for all subjects at each wavelength was as follows: 325 nm, 0·64 (SD 0·37); 335 nm, 0·80 (SD 0·58); 345 nm, 0·96 (SD 0·55); 355 nm, 1·50 (SD 0·85); 365 nm, 2·19 (SD 0·90); and 375 nm, 2·89 (SD 1·06). Therefore, the relative sensitization at each wavelength (erythemal action spectrum) was: 1, 0·83, 0·67, 0·43, 0·29 and 0·22. There were significant differences between the PUVA erythemal effectiveness at different wavelengths but none between skin types. CONCLUSIONS: This study has established the erythemal action spectrum for bath/soak PUVA therapy as is currently performed. In all volunteers, the peak sensitivity was at 325 nm. All volunteers showed a similar trend across the wavelengths studied irrespective of skin type. The determination of the action spectrum for PUVA-induced erythema is important as it permits reliable estimates of erythemal efficacy of any UVA source where the emission spectrum of the lamp is known or can be measured.