STRC Research

Cochlear Drug Delivery Gradients

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Cochlear Drug Delivery Gradients

There is no such thing as a uniform drug distribution in the cochlea from round window delivery. This isn’t a limitation to overcome with technique — it’s physics.

Diffusion is slow. Clearance from scala tympani is fast (t½ ~60 min). These two forces create a standing concentration gradient from base to apex that persists indefinitely. Salt & Ma 2001 calculated it in a simulator and verified it in live guinea pigs: basal turn concentration is 40× higher than apical after steady-state.

What this means for gene therapy:

  • Basal OHCs (high-frequency hearing, 8-16 kHz range) get the most vector copies.
  • Apical OHCs (low-frequency, speech range) get far less.
  • If your therapeutic dose is calibrated for the base, the apex is undertreated. If calibrated for the apex, the base may be overtreated and toxic.

Workarounds used in practice:

  • Posterior semicircular canal injection (PSCC): delivers AAV closer to the apex. Used in Zhao et al. 2025 for tree shrew experiments and in some clinical trials.
  • Cochleostomy: direct access but risks trauma.
  • Higher total dose: brute-force but increases off-target liver/brain transduction.
  • Longer injection time / hydrogel depot: slows clearance, may improve distribution slightly.

Molecular size matters too. Larger molecules (e.g., AAV capsids, ~25 nm diameter) diffuse slower than small ions like TMPA. The Salt & Ma numbers are for small ions — actual AAV gradients are steeper.

The human cochlea is 28.5 mm long vs 4.5 mm in mice. In mice, a drug spreading a few mm reaches most of the cochlea. In humans, it barely covers the base. Every mouse result needs a cochlear length correction before claiming clinical relevance.

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