Bisphosphonate-Linked TrkB Agonist: Cochlea-Targeted Delivery of a Neurotrophic Agent as a Strategy for the Treatment of Hearing Loss

Abstract

Hearing loss affects more than two-thirds of the elderly population, and more than 17% of all adults in the U.S. Sensorineural hearing loss related to noise exposure or aging is associated with loss of inner ear sensory hair cells (HCs), cochlear spiral ganglion neurons (SGNs), and ribbon synapses between HCs and SGNs, stimulating intense interest in therapies to regenerate synaptic function. 7,8-Dihydroxyflavone (DHF) is a selective and potent agonist of tropomyosin receptor kinase B (TrkB) and protects the neuron from apoptosis. Despite evidence that TrkB agonists can promote survival of SGNs, local delivery of drugs such as DHF to the inner ear remains a challenge. We previously demonstrated in an animal model that a fluorescently labeled bisphosphonate, 6-FAM-Zol, administered to the round window membrane penetrated the membrane and diffused throughout the cochlea. Given their affinity for bone mineral, including cochlear bone, bisphosphonates offer an intriguing modality for targeted delivery of neurotrophic agents to the SGNs to promote survival, neurite outgrowth, and, potentially, regeneration of synapses between HCs and SGNs. The design and synthesis of a bisphosphonate conjugate of DHF (Ris-DHF) is presented, with a preliminary evaluation of its neurotrophic activity. Ris-DHF increases neurite outgrowth in vitro, maintains this ability after binding to hydroxyapatite, and regenerates synapses in kainic acid-damaged cochlear organ of Corti explants dissected in vitro with attached SGNs. The results suggest that bisphosphonate-TrkB agonist conjugates have promise as a novel approach to targeted delivery of drugs to treat sensorineural hearing loss.

Regeneration of Cochlear Synapses by Systemic Administration of a Bisphosphonate

“…The kainate excitotoxicity results also show that the drug is effective in restoring synapses damaged by glutamate toxicity, which is thought to be the mechanism of noise damage as well (Liberman and Kujawa, 2017). Consistent with our previous description of zoledronate's direct action on spiral ganglion cells (Schaette and McAlpine, 2011), another study found an effect of a related bisphosphonate, risedronate, in vitro (Kempfle et al, 2018). However, that study was focused on the synthesis of a risedronate conjugate with a neurotrophic agent to deliver the agent to cochlear neurons via risedronate's binding to the surrounding bone.…”

“…Intracellular signaling elicited by neurotrophins is mediated in part by protein prenylation ( Li et al, 2013 ), suggesting a link between neurotrophins and bisphosphonates. While Kempfle et al (2018) found that 400 nM risedronate alone had a minor positive effect on the growth of neurites in vitro , we found a much more robust effect with 1 μM zoledronate on promoting neurite extension and synapse regeneration in vitro and in vivo . Differences in the magnitude of the observed effect may reflect differences in drugs’ neuroregenerative potential, drug concentration, model systems, and research foci of the two studies.…”

“…The postulated mechanism for disruption of synapses by noise involves excessive release of glutamate into the synaptic cleft with attendant damage to the postsynaptic neurons ( Liberman and Kujawa, 2017 ). We used an in vitro model of excitatory cochlear synaptopathy that we have employed previously ( Wang and Green, 2011 ; Kempfle et al, 2018 ; Yamahara et al, 2019 ) to further probe the mechanism of the regenerative effect of the drug on cochlear synapses. In this model, cochlear explants consisting of hair cells and attached SGNs are exposed to kainic acid (KA), a selective ionotropic glutamate receptor agonist.…”

Cochlear protein biomarkers as potential sites for targeted inner ear drug delivery

“…Recent work has demonstrated that nanoparticles targeted with the TrkB binding A371 peptide bind preferentially to the spiral ganglion's neuronal cells [35], and that this targeting peptide has enabled Rolipram, a phosphodiesterase inhibitor, to be delivered to the mouse spiral ganglia as a potential agent that improves cell survival [35,45]. Additional work has suggested the possibility of targeted delivery of the TrkB agonist, 7,8-dihydroxyflavone (DHF) as a means for increasing neurite growth in the setting of kainicacid cochlear damage [46]. Similarly, targeted delivery of [41,42] neural growth factors such as BDNF [47] may offer the potential for neural regeneration.…”

Synthesis of carboxy‐polyethylene glycol‐amine (CA (PEG)n) and [1‐14C]‐CA (PEG)n via oxa‐Michael addition of amino‐polyethylene glycols to propiolates vs to acrylates

“…Carboxy‐polyethylene glycol‐amine (CA (PEG) n, 1 ) compounds are pegylated amino acids with a short and well‐defined PEG spacer between the carboxy and amino termini (Figure ). CA (PEG) n has been used as bifunctional crosslinkers in constructing bioconjugates for targeted drug delivery,[1a, b] targeted radionuclide therapy and imaging, targeted protein degradation, and bivalent modulator design [4a, b]. Radiolabeled bioconjugates provide an understanding of their pharmacokinetics and biodistribution during therapeutic development; however, unlike small molecules, introducing radiolabels into biologics remains a formidable challenge.…”