Effects of Sequential Ultrasound Irradiation in The KHz and MHz Bands on Transdermal Administration of Biopolymeric Drugs

<b>Introduction</b><br/>Biopolymeric drugs are expected to have high efficacy and minimal side effects due to their specificity. Meanwhile, a minimally invasive method of administering these drugs is difficult to establish. As one of the attracting methods, sonophoresis is a method of drug-administering by disrupting the stratum corneum layer through the collapse of cavitation bubbles and drug infusion by the microjets induced by the bubbles (N. Deagicevic, <i>et al</i>., <b><i>Springer</i></b> 2017). However, the amount of drugs administered by conventional techniques is limited, since the randomness of the microjets generated from cavitation bubbles makes stable drug infusion difficult. This is because the position of the bubble nucleus for the generation of cavitation bubbles is unstable and not directional.<br/>Here, we propose an ultrasound irradiation method with multiband to achieve efficient transdermal administration of biopolymeric drugs. Especially, the synergistic effect of cavitation induced by low-frequency and acoustic streaming induced by high-frequency on drug administration is demonstrated by sequentially irradiating in the kHz and MHz bands with fluorescence-modified ovalbumin (OVA, for nano-sized drug model).<br/><br/><b>Materials and methods</b><br/>Transdermal administration experiments with fluorescence-modified OVA to porcine skin irradiated by ultrasound (US) were carried out on the effectiveness of acoustic streaming by sequential irradiation in the kHz and MHz bands. Here, the irradiation distance of the acoustic streaming on the drug dosage was evaluated. Ultrasound irradiation was experimented under the following four conditions. As a conventional method (i), US in the kHz band was applied for 10 min (control). (ii)-(iv) As the method proposed, US in the kHz band and MHz band was sequentially applied for 5 min., respectively. The MHz band was irradiated at a distance of (ii) 1 mm, (iii) 2 mm, and (iv) 3 mm. After irradiation of multiband US, the skin from each condition was lysed, and the fluorescence intensity of cyanine 5 modified OVA was measured by a spectrofluorometer to evaluate the dose of the nano-drug model quantitatively.<br/><br/><b>Results and discussion</b><br/>In order to investigate the effect of acoustic streaming on the proposed method, the ultrasonic irradiation device was constructed using two types of transducers. A Langevin transducer with a resonance frequency of 45.9 kHz was used for kHz band irradiation (X. Xie, <i>et. al.</i>, <b><i>J Drug Deliv Sci Technol</i></b> 2022), and a piezoelectric device with a resonance frequency of 1.97 MHz was utilized for MHz band irradiation. The velocity of acoustic streaming is expressed as a function that depends on the distance from the irradiation source (J. Friend <i>et. al</i>, <b><i>Lab Chip</i></b> 2019). The results obtained from the function suggest that under the conditions of this study, the flow velocity rises with increasing irradiation distance in the MHz band. This calculated outcome shows that the drug dosage increases in the range of 1 mm to 3 mm. The results of fluorescence intensity measurements for each condition suggested that the highest drug dose was obtained when the US in the MHz band was irradiated at an irradiation distance of 3 mm after irradiation of the US in the kHz band. Furthermore, as with the theoretical value, the drug dose significantly rose with increasing irradiation distance ((i) vs (iv): *<i>p</i>&lt;0.05, (ii) vs (iv): **<i>p</i>&lt;0.01, Student's <i>t</i>-test). These results suggest that sequential irradiation of the kHz and MHz bands is more effective than conventional irradiation of only the kHz band. Therefore, the acoustic streaming induced by the MHz band contributes to the improvement of drug dosage for sonophoresis.<br/>The development of the multiband US administration method enables us to propose minimally invasive biomacromolecule drug administration as an alternative to injection. Therefore, this method is expected to contribute to the development of research on biopolymeric drug materials.