Verena Santer, Sergio del Río Sancho, Yogeshvar N. Kalia*
School of Pharmaceutical Sciences,
University of Geneva & University of Lausanne, CMU, 1 rue Michel Servet
1211 Geneva 4, Switzerland
Abstract
Hypertrophic and keloid scars, despite not being life-threatening conditions, are a major concern in dermatology given their deforming and emotional load for patients. The common first line treatments are corticosteroid injections, mainly triamcinolone acetonide (TA) (10-40 mg/ml); however, given the painfulness of the procedure and the risk of severe side effects, alternatives would be desirable. In this study the localized iontophoresis of triamcinolone acetonide amino acid (TA-AA) prodrugs synthesized in-house was tested as a potential means for a non-invasive treatment option. Delivery of alanine, arginine, glycine and lysine TA prodrugs (TA-Ala, TA-Arg, TA-Gly and TA-Lys) was quantified in vitro on porcine and human skin, following 20 min anodal iontophoresis at 0.5 mA/cm2. In both skin models, 2-fold higher TA deposition following TA-Ala and TA-Gly iontophoresis was found as compared to TA-Arg and TA-Lys.
It was supposed that the double positive charged prodrugs (TA-Arg and TA-Lys), given their localized charge and lipophilic backbone, were interacting more with the skin, which reduced iontophoretic delivery.
Experiments of the effect of TA-Arg and TA-Lys on the permselectivity of the skin and lidocaine (LID) electromigration reinforced this hypothesis. Nevertheless, when dermal biodistribution profiles were investigated by the horizontal slicing technique, all prodrugs delivered supra-therapeutically amount of TA in the depth of 1 mm. In the second part of the study co-iontophoresis of verapamil (VER) and TA-Arg was
investigated, since synergic effects in the treatment of hypertrophic and keloid scars had been suggested by the combination of the calcium antagonist with the corticosteroid. Co-iontophoresis of TA-Arg (0.5 mM) and VER (9.5 mM) showed therapeutically effective concentrations (VER) in the entire skin specimen of 1 mm, whereas TA concentration was reduced and more superficial. These preliminary results suggest that co-iontophoretic application could be a valid option for effective local treatment minimizing the risk of increased corticosteroid exposure by chronically repeated medication.
Keywords: triamcinolone acetonide, prodrugs, verapamil, topical iontophoresis, keloid and hypertrophic scars
Introduction
Iontophoresis is a non-invasive drug delivery method, which exploits a low current for the transport of active principles into and through biological membranes [1] [2]. Successful transdermal drug delivery of small molecules [3] [4] [5] and high molecular weight proteins [6] [7] [8] were repeatedly shown in the past. The best suited drugs for the iontophoretic transport are usually water soluble and charged; however only few correspond to these criteria. Thus, one approach to optimize drug molecules physicochemical properties for the iontophoretic transport is to temporarily modify their molecular structure as labile prodrugs (e.g. esterification with amino acid (AA) side chains).As example: the transdermal delivery of dehydroepiandrosterone (DHEA) was found superior to passive via iontophoresis of its glycine ester prodrug; moreover the complete conversion of the AA-prodrug to the parent active molecule was seen in the tissue [9]. In another study glycine, proline, alanine and valine ester prodrugs of a dopamine agonist were synthesized and tested for transdermal iontophoresis in vitro and in vivo. Despite an increase in delivery was observed, transdermal permeation of the active principle was shown delayed most probably because of the prompt hydrolysis of the ester [10]. Therefore, it was concluded that the use of labile prodrugs was especially suited for increased topical delivery. This was shown with the delivery of valaciclovir [11] and other AA prodrugs of aciclovir [12] [13]. The combination of prodrugs to anodal iontophoresis enabled the deposition of supra-therapeutic amounts of aciclovir in the basal epidermis, which is also the main target area for the treatment of topical herpes simplex infections [13].
In the following study the combination of in house synthesized AA ester prodrugs of triamcinolone acetonide (TA-AA) and anodal iontophoresis was investigated for the topical and non-invasive treatment of hypertrophic and keloid scars. These local conditions are characterized by excessive scar tissue formation which have functional and mostly severe emotional implications for the patients [14] [15]. The underlying process was suggested to be a pathological persistence of wound healing signals which lead to excessive scar formation following trauma in predisposed people [16]. The outcome are protruded, often
hyperpigmented scar tissues, which in case of hypertrophic scars remain confined to the original wound area and eventually regress spontaneously [15] [16] [17]. Keloid scars on the contrary, are more insidious since the scar tissue overgrows the initial area of the trauma and they do not regress spontaneously, additionally the patients suffer from pruritus and pain on the affected sites [14] [18].
The golden standard of treatment are monthly corticosteroid injections, mostly TA suspension (10-40 mg/ml Kenalog) [14] [19]. The effect of corticosteroids is the inhibition of fibroblast proliferation, collagen synthesis and anti-inflammatory, which also reduce the pain and itchiness of the lesion [15] [19]. However, the repeated injections were stated to be very painful for the patients and long term use over a large area can cause severe local damage such as: skin depigmentation, fat atrophy, telangiectasia and ulceration [14]
[19] [20] and systemic side effects, such as Cushing syndrome [21] [22]. Also, recurrence of the dermal hyperproliferation is common and therefore corticosteroid therapy is combined with physical approaches such as cryotherapy, silicon pressure therapy or surgical excision. Less common are the combination with other active principles such as immunosuppressant or calcium channel blockers [14] [15].
Calcium channel blockers such as verapamil (VER) were shown to change fibroblasts cell shape, trigger the collagenase production and inhibit the synthesis of extracellular matrix [23] [24] [25]. The treatment of hypertrophic and keloid scars with VER injections was tested with success, although studies were performed on limited number of subjects [26] [27] [28]. The iontophoretic delivery of VER has been already investigated for the treatment of Peyronie’s disease, given the its high aqueous solubility and the positive charge at physiological pH [29] [30] [31]. In the case of hypertrophic and keloid scars the combination[32] [27] or alternation of TA with VER were suggested, given the different mechanism of action of the two principles on the induction of collagenase [15] [20] [33].
In order to propose a non-invasive treatment option for hypertrophic and keloid scars in this study i) the iontophoretic delivery of novel TA-AA prodrugs, ii) influence of AA sidechains, application time and current intensities as well as ii) the co-iontophoretic treatment of TA-AA with VER were investigated.