Toxicity of pyrithiones

Seborrheic dermatitis: topical therapeutics and formulation design

Seborrheic dermatitis (SD) is a common dermatological disorder with symptoms that include skin flaking, erythema and pruritus. This review discusses the topical products available for treating SD, which target several aspects of disease pathobiology, including cutaneous microbial dysbiosis (driven by Malassezia yeast), inflammation, sebum production and skin barrier disruption. Among the various treatments available, zinc pyrithione (ZnPT) based products that exhibit anti-fungal action are the market leaders. A skin compartment approach is presented here for combining ZnPT exposure information with threshold levels for anti-fungal efficacy and toxicity, overall providing a comprehensive picture of ZnPT therapeutics and safety. While Malassezia yeast on the surface are effectively targeted, yeast residing beyond the superficial follicle may not receive adequate ZnPT for anti-fungal effect forming the basis for skin re-colonisation. Levels entering systemic circulation from topical delivery are well below toxic thresholds, however the elevated zinc levels within the viable epidermis warrants further investigation. Strategies to improve formulation design can be broadly classified as influencing 1) topical delivery, 2) therapeutic bioactivity, 3) skin mildness, and 4) sensory attributes. Successful SD treatment ultimately requires formulations that can balance efficacy, safety, and consumer appeal.

A Physiological-Based Pharmacokinetic Model For The Broad Spectrum Antimicrobial Zinc Pyrithione: II. Dermal Absorption And Dosimetry In The Rat

The broad spectrum antimicrobial/antifungal zinc pyrithione (ZnPT) is used in products ranging from antifouling paint to antidandruff shampoo. The hazard profile of ZnPT was established based upon comprehensive toxicological testing, and products containing this biocide have been safely used for years. The purpose of this study was to create a dermal physiologically based pharmacokinetic (PBPK) model for ZnPT in the rat for improving dose-response analysis of ZnPT-induced toxicity where reversible hindlimb weakness was the endpoint used as the basis for ZnPT risk assessments. Previously, we developed a PBPK model which simulated the kinetics of pyrithione (PT) and its major metabolites 2-(methylsulfonyl)pyridine and S-glucuronide conjugates in blood and tissues of rats following oral ZnPT administration. The dermal model was optimized utilizing in vitro dermal penetration investigations conducted with rat skin and with historical data from a dermal repeat dose study using rats. The model replicated the observed temporal patterns and elimination kinetics of [¹⁴C]PT equivalents in blood and urine during and following repeated dermal dosing and replicated the observed dose-dependencies of absorption, blood [¹⁴C]PT equivalents and plasma PT concentrations. The model provided internal dosimetry predictions for a benchmark dose analysis of hindlimb weakness in rats that combined dermal, gavage and dietary studies into a single internal dose-response model with area-under-the-curve (AUC) for plasma PT, the toxic moiety in the rat, as the internal dose metric. This PBPK model has predictive validity for calculating internal doses of PT and/or [¹⁴C]PT equivalents from different routes of exposure in the rat.

Formulation and Artificial Sebum Effects on the Percutaneous Absorption of Zinc Pyrithione through Excised Human Skin

BACKGROUND

Zinc pyrithione (ZnPT) is deposited on the skin as a fine particulate and must reach microorganisms localized in the stratum corneum and hair follicles in molecular form to exert its broad-spectrum antimicrobial/antifungal activity. Dissolution of ZnPT particles followed by molecular speciation results in the organic portion, i.e. pyrithione, being more susceptible to skin penetration than the inorganic component, i.e. zinc, or the chelate itself, i.e. ZnPT.

OBJECTIVES

To further test the hypothesis that ZnPT skin penetration is rate-limited by dissolution and molecular speciation, the effect of different formulations and artificial sebum on the in vitro percutaneous absorption of radiolabel associated with Zn[14C]PT was investigated.

METHOD

In vitro penetration of [14C]PT into and through excised human skin was measured following application of Zn[14C]PT prepared as suspensions in distinct vehicles including water-based carboxymethylcellulose (CMC), diluted body wash comprised of surfactants, and castor oil, in the presence and absence of artificial sebum.

RESULTS

The steady-state flux and cumulative absorption of Zn[14C]PT increased 4- to 5-fold when deposited from a body wash or castor oil compared to a water-based CMC suspension. Tritiated water flux measured before and after treatment showed that neither the surfactant vehicle nor castor oil significantly altered barrier function versus water alone. An artificial sebum layer on the skin potentiated Zn[14C]PT and 3H2O absorption when dosed from both aqueous formulations, but not from castor oil.

CONCLUSION

These data are consistent with the hypothesis that ZnPT percutaneous absorption, as measured by [14C]PT kinetics, is controlled by particle dissolution and molecular speciation.

A physiologically based pharmacokinetic model for the broad-spectrum antimicrobial zinc pyrithione: I. Development and verification

The broad-spectrum antimicrobial zinc pyrithione (ZnPT) is used in numerous products ranging from in-can preservative/mildicide in paints to antidandruff shampoo. Although products containing ZnPT have a long history of safe use, regulatory agencies routinely set limits of exposure based upon toxicological considerations. The objective of this study was to create a physiologically based pharmacokinetic (PBPK) model for ZnPT in the rat for improving dose-response analysis of ZnPT-induced toxicity, reversible hindlimb weakness, the endpoint that has been used as the basis for ZnPT risk assessments. A rat oral PBPK model was developed that includes compartments for plasma, liver, kidneys, muscle, brain, and rapidly and slowly perfused tissues. Pyrithione metabolism to 2-(methylsulfonyl)pyridine (MSP) and glucuronide conjugates was incorporated into the model. The model was parameterized and optimized based upon data from single-dose intravenous (iv) and oral gavage pharmacokinetic studies of radiolabeled pyrithione ([¹⁴C]PT) administered as zinc [¹⁴C]-pyrithione (Zn-[¹⁴C]PT) to adult female rats. It was further evaluated and refined using data from repeated, multidose oral gavage and dietary studies of Zn[¹⁴C]PT in the adult female rat that included measurements of plasma PT concentration, the putative toxic species. The model replicated the observed short-term elimination kinetics of PT in plasma and [¹⁴C]PT in whole blood following single doses and longer term temporal patterns of plasma and blood concentrations during repeated dosing schedules. The model also accounted for production and rapid elimination of S-glucuronide conjugates (SG) of 2-pyridinethiol and 2-pyridinethiol-1-oxide in urine, as well as production and slower elimination of MSP, the major [¹⁴C]PT species in blood within several hours following administration of ZnPT. The model provided internal dosimetry predictions for a benchmark dose (BMD) analysis of hindlimb weakness in rats, and was used to combine gavage and dietary studies into a single internal dose-response model with area under the curve (AUC) for plasma PT as the internal dose metric. This PBPK model has predictive validity for calculating internal doses of PT and/or [¹⁴C]PT from different routes of exposure in the rat.

Zinc pyrithione in alcohol-based products for skin antisepsis: persistence of antimicrobial effects

Alcohol-based products for skin antisepsis have a long history of safety and efficacy in the United States and abroad. However, alcohol alone lacks the required antimicrobial persistence to provide for the sustained periods of skin antisepsis desired in the clinical environment. Therefore, alcohol-based products must have a preservative agent such as iodine/iodophor compounds, chlorhexidine gluconate, or zinc pyrithione, to extend its antimicrobial effects. Iodine, iodophors, and chlorhexidine gluconate are well-characterized antimicrobials and preservatives. The thrust of our effort was to examine the characteristics of the lesser-known zinc pyrithione and to evaluate its utility as a preservative in the formulation of alcohol-based products for skin antisepsis. This work includes a literature review of current zinc pyrithione applications in drugs and cosmetics, a safety and toxicity evaluation, consideration of the proposed mechanisms of antimicrobial action, in vitro and in vivo efficacy data, and a discussion of the mechanisms that confer the desired antimicrobial persistence. In addition, alcohol-based, zinc pyrithione-preserved, commercially available products of skin antisepsis are compared with other commercially available antimicrobials used for skin antisepsis and with additional alcohol-based products with different preservatives. The authors' conclusion is that zinc pyrithione is not only a safe and effective antimicrobial but that its use in certain alcohol-based formulations results in antimicrobial efficacy exceeding that of iodine and chlorhexidine gluconate.

Chromatographic behavior of pyrithiones

Pyrithione biocides are currently viewed as a major prospect for the replacement of tributyltin antifoulants in ship paints. The chromatographic behavior of 1-hydroxy-2-pyridinethione (pyrithione, PT), bis(2-pyridinyl)disulfide 1,1'-dioxide (PT2), and the metal complexes zinc [Zn(PT)2], iron [Fe(PT)3] and copper [Cu(PT)2] pyrithione, were investigated by means of UV-vis spectroscopy, ESI-MSn, HPLC-DAD and HPLC-ESI-MSnD. This revealed transformations of the analytes, which affect the development of adequate methods for species or environmental analysis of pyrithiones. PT transforms into copper- or iron- containing complexes and/or the oxidation product PT2, depending on the type of the stationary phase used in chromatographic analysis. Speciation complicates direct chromatography of [Zn(PT)2] and [Fe(PT)3].

Effect of sodium pyridinethione on the uptake and distribution of nickel in rats, ferrets and guinea-pigs

Oral administration of sodium pyridinethione together with Ni2+ (using 63Ni2+ as a tracer) to rats, ferrets and guinea-pigs produced highly increased tissue levels of the metal in several tissues in comparison with animals given the Ni2+ alone. Ni2+ forms a lipophilic complex with pyridinethione and it can be assumed that a facilitated passage of the Ni2+ across the cellular membranes of various tissues is important for the observed effects. Pigmented tissues (e.g. the eye melanin), the pancreatic islets, the nervous system and striated muscles showed high levels of Ni2+ in animals given sodium pyridinethione. However, in some instances marked species differences were observed. Thus, microautoradiography indicated an uptake of Ni2+ both in the beta- and alpha-cells in the pancreatic islets in the rat, whereas in the guinea-pig only some cells (probably the alpha-cells) accumulated high levels of Ni2+. In the ferret sodium pyridinethione induced a high uptake of Ni2+ in the heart muscle, which was not seen in the other species. The Ni2+ is probably taken up in the various tissues complexed to pyridinethione. Within the tissues the complex may dissociate and the Ni2+ may bind to some endogenous tissue components. The affinity of the Ni2+ for the endogenous ligands in relation to the affinity for the pyridinethione may be of importance for the effects on the disposition of the Ni2+. The species variations may be related to differences in the structural conformations of the endogenous Ni(2+)-binding ligands.

Percutaneous absorption of Octopirox

[14C]Octopirox administered to rats by intubation or injection was excreted mostly in the faeces (65-85% of the dose) with smaller amounts (6-19%) in the urine. Blood levels after intubation of Octopirox (4.8 mg/kg body weight) reached a maximum equivalent to 0.137 micrograms/ml at 2 hr and declined to 0.007 micrograms/ml at 48 hr after administration. Tissue levels were low, the greatest was the liver with the equivalent of 3 micrograms Octopirox at 6 hr after intubation. With female rats skin penetration of Octopirox at 1% (v/v) in shampoo without rinsing was 65.1 micrograms/cm2 under non-occlusive conditions for 48 hr. When the skin was rinsed after a 10-min contact, penetration was reduced to 3.4 micrograms/cm2 under occlusive, and 2.0 micrograms/cm2 under non-occlusive conditions. Skin penetration of Octopirox was dependent on duration of contact up to 10 min before rinsing. Penetration at 1% Octopirox increased significantly from 2.4 micrograms/cm2 after 2.5 min exposure to 4.5 micrograms/cm2 after 10 min contact, but there was no further increase in penetration with a 20-min application. Skin penetration and deposition of Octopirox were both proportional to Octopirox concentration between 0.1 and 1% (w/v); skin penetration increased from 0.31 to 3.6 micrograms/cm2 while deposition increased from 0.8 to 7.6 micrograms/cm2. There was no significant difference between the penetration through clipped skin and hairy skin from an application of 1% Octopirox for 5 min followed by rinsing. Under non-occlusive conditions, penetration was 1.5 micrograms/cm2 for both types of skin. Blood levels after topical application (15.4 mg/kg body weight) without rinsing and with occlusion reached the equivalent of 0.32 micrograms/ml at 6 hr. However, when the skin was rinsed and protected with a non-occlusive patch blood levels were reduced to a maximum equivalent to 0.02 micrograms/ml at 1 hr after application. The safety factor estimated for the consumer using a shampoo containing 1% Octopirox is 29,400, so that the possibility of systemic effects due to absorption through the skin is remote.