Liquid loaded microneedles for the intradermal deliveryof botulinum toxin for Primary Focal Hyperhidrosis

Primary focal hyperhidrosis (PFHH) is a medical condition characterised by overactivity of the eccrine sweat glands, primarily occurring on palmar, plantar and axillary regions. PFHH can have a significant adverse impact on a patient’s quality of life. Multiple intradermal injections of a commercial formulation of botulinum toxin A (BTX A) (Botox®) is the most effective non-surgical treatment currently licensed in the UK for cases of severe PFHH. Although effective, intradermal BTX A injections are associated with considerable pain and discomfort for the patient and are time-consuming for the administering clinician. This study aims to evaluate the potential of using pocketed microneedle devices for minimally invasive intradermal delivery of BTX A, as a liquid formulation, into human skin. Pocketed microneedles, metallic 700 μm-long needles containing a cavity within the needle shaft, were selected as an appropriate and relatively untested intradermal delivery device. Pocketed microneedle devices (PMDs) were liquid loaded by immersion into a ‘Botox® like’ formulation that mimicked the composition of the commercial Botox® formulation, with the exception of BTX A, which was replaced by the model macromolecular protein β-galactosidase (~465 kDa). A water-soluble dye was also included to enable visualisation. Microneedles were assessed for loading uniformity by light microscopy and the formulation residency time was evaluated by monitoring evaporation using a digital camera. The microneedle loading capacity was determined using an established quantitative assay for β-galactosidase. Studies using excised human breast skin, maintained in organ culture, examined delivery of the model β-galactosidase from liquid loaded PMDs and the time-dependent diffusion of the protein within the dermal tissue. A more clinically representative model of BTX A, formaldehyde inactivated BTX A, i.e., botulinum toxoid, was used to determine the deposition pattern of the therapeutic within the skin. Following skin delivery the toxoid was detected by immnohistochemical staining and fluorescence imaging, following its conjugation to an appropriate fluorophore. Immersion of the PMD into a ‘Botox® like’ formulation resulted in successful uptake and retention of the model protein solution. Quantitative studies indicated that nanogram quantities (~100 ng/microneedle array) of the β-galactosidase model can be loaded and retained on individual microneedles, in a liquid state. These results suggest that the loading capacity of the microneedle device is appropriate for therapeutic botulinum toxin formulations, although loading uniformity will need to be addressed. Histological analysis revealed effective delivery of the model β-galactosidase from a PMD to the epidermal and the dermal layers of the skin. Rapid and extensive diffusion of the protein within the deeper dermis was also demonstrated. Further, immunohistochemical and fluorescence studies indicated effective PMD loading and successful delivery of botulinum toxoid to the dermis of human skin. These data suggest that it should be possible for BTX A to access its therapeutic target (the eccrine sweat glands) following delivery via PMDs. This study has demonstrated for the first time that pocketed microneedles represent a viable, minimally invasive alternative, for the intradermal delivery of botulinum toxin A (Botox®). Future pre-clinical and clinical studies are now required to test and optimize a microneedle-based delivery system that is most suited to clinical practice.