Single Application Cold-Chain Independent Drug Delivery System for Outer Ear Infections

Abstract

Outer ear infections (OE) affect millions of people annually with significant associated healthcare costs. Incorrect administration or non-compliance with the treatment regimen can lead to infection persistence, recurrence, antibiotic resistance, and in severe cases aggravation to malignant otitis externa. Such issues are particularly pertinent for military personnel, patients in nursing homes, the geriatric population, for patients with head or hand tremors and for those with limited or no access to proper healthcare. With the intent of using traditional material science principles to deconvolute material design while increasing relevance and efficacy, we developed a single application, cold-chain independent thixotropic drug delivery system. This can be easily applied into the ear as a liquid, then gels to deliver effective concentrations of antibiotics against bacterial strains commonly associated with OE. The system maintains thixotropic properties over several stress/no stress cycles, shows negligible swelling and temperature dependence, and does not impact the minimum inhibitory concentration or bactericidal effects of relevant antibiotics. Moreover, the thixogels are biocompatible and are well tolerated in the ear. This drug delivery system can readily translate into a user-friendly product, could improve compliance via a single application by the diagnosing health care provider, is expected to effectively treat OE and minimize the development of antibiotic resistance, infection recurrence or exacerbation.

Figure 1.. Empirical evaluation of thixogels.

A – the University of Montana initials (UM) drawn by deploying our H-Tx thixogel though a 26G 1/2 needle (right). B – the thixogel liquefies when pushed through the needle/syringe nozzle and rapidly gels once deployed.

Figure 2.. Rheological evaluation of thixogels.

A – thixotropy evaluation of H-Tx. B – thixotropy evaluation of S-Tx. Blue lines indicate storage moduli (G’) and green lines indicate loss moduli (G”). C – temperature dependence of H-Tx. D – temperature dependence of S-Tx. The lines indicate storage moduli (G’).

Figure 3.. Antibiotic release profiles from thixogels.

A – effects of H-Tx and B – S-Tx on the kill kinetics of ciprofloxacin against S. aureus 13709 and P. aeruginosa 27853. The MIC for ciprofloxacin is 0.25 μg/mL (from Table 2); the 30 μg/mL is a dose that is ~100 times lower than the intended clinical dose of ciprofloxacin (3,000 μg/mL or 0.3% w/v). Growth was measured by sampling media placed above the thixogel and then quantifying colony forming units (CFU’s) by serial dilution. C – Ciprofloxacin release profiles from H-Tx and S-Tx. Both thixogels released the loaded drug at comparable rates via diffusion (n = 3). The equilibrium concentration between gels and supernatant saline was reached within 24 hours.

Figure 4.. Biocompatibility evaluation of thixogels.

A – cytocompatibility of thixogel formulations as determined by a colorimetric methyltetrazolium assay (MTT) with primary human dermal fibroblasts (n = 3). Cells with no treatment on tissue culture plate (TCP) were used as control. The dotted red line indicates the cut-off for cytocompatibility per ISO 10993–5. ANOVA analysis: p = 0.013 for alpha = 0.05, F = 9.61, F crit = 5.14, DF = 3. B – standardized skin irritation test with EpiDerm™ indicating that the thixogels are non-irritant per UN GHS (n = 3). NC – negative control. An irritant is defined as a test substance that reduces tissue viability ≤ 50% of the mean viability (dotted red line) of the negative controls (NC); NC – Dulbecco’s Phosphate Buffered Saline; PC – 5% Sodium Dodecyl Sulfate solution. ANOVA analysis: p = 4.41E-06 for alpha = 0.05, F = 19.07, F crit = 3.09, DF = 3. C – skin corrosion testing of thixogels with EpiDerm™. After 3 minutes of exposure to HA or SF formulations, the viability of tissues was above the 50% acceptance threshold (red dotted line). ANOVA analysis: p = 2.25E-31 for alpha = 0.05, F = 8814.54, F crit = 3.09, DF = 3. D – skin irritation test after the 1-hour exposure, tissues treated with both formulations fully recovered and had better viability that the negative control (NC) H₂O treated tissues. As positive control (PC) an 8N KOH solution was used (acceptance threshold after 1 hour is ≥ 15%, indicated by the red dotted line). ANOVA analysis: p = 8.21E-23 for alpha = 0.05, F = 1221.19, F crit = 3.09, DF = 3.

Figure 5.. Physiological effects of thixogels of thixogel application into ears.

Top panels illustrate the otoscopic evaluation of the mouse ear canal: A – prior to thixogel application; B – immediately after thixogel application (thixogel highlighted by dotted area and indicated by the arrow); and C – 10 days post deployment. No detectable thixogel was present 10 days post-deployment. The bottom row illustrates the histological evaluation of tissue treated with D – saline (control); E – H-Tx and F – S-Tx. Scale bar is 100 μm.

Figure 6.. Effects of thixogel topical application on hearing.

ABR measurements of hearing upon application of equivalent volumes of A – saline (control); B – HA thixogel; and C – SF thixogel in the outer ear canal. DPOAE measurements of hearing upon application of equivalent volumes of D – saline (control); E – HA thixogel; and F – SF thixogel in the outer ear canal. CTRL = measurements recorded prior to administration of saline, H-Tx or S-Tx, respectively.