Selank is one of the few research peptides where the nasal spray is not a convenience format retrofitted onto an injectable compound. The Russian group that developed it studied it intranasally, and the pharmacokinetic work that exists was done through that route. That makes the route comparison unusual: here the better-characterised side is the spray.
The molecule
Selank is a heptapeptide, Thr-Lys-Pro-Arg-Pro-Gly-Pro, built from the tetrapeptide tuftsin with a Pro-Gly-Pro tail attached. That tail is the design feature. Short native peptides are destroyed in plasma within minutes; the C-terminal Pro-Gly-Pro extension was added to slow enzymatic degradation of the tuftsin fragment. Understanding the route behaviour means understanding what that modification does and does not buy.
What the labelling work measured
Zolotarev and colleagues published the relevant study in 2006, using peptides evenly labelled with tritium at 50 to 150 Ci/mmol prepared by high-temperature solid-phase catalytic isotope exchange. Even labelling across all amino acid residues is what makes the method useful: it allows simultaneous determination of essentially every enzymatic hydrolysis product, not just the intact peptide.
Applied to Selank, the work covered biodegradation in blood plasma and, separately, pharmacokinetics in brain tissue following intranasal administration in vivo. The major degradation products were identified as the pentapeptide TKPRP, the tripeptide TKP, and the dipeptides RP and GP.
That degradation map is the substance of the result. Selank in tissue is not a single species but a decaying cascade, and the fragments appear quickly. Whatever the compound is doing, a measurement of "Selank concentration" at any timepoint is measuring one member of a mixture.
Why the route matters more here than usual
Selank's proposed activity is central: the published interest is anxiolytic. For a compound aimed at the brain, the two routes are not two ways of achieving the same exposure. An injection must deliver the peptide into circulation and then across the blood-brain barrier, which short peptides cross poorly. Intranasal administration offers the olfactory and trigeminal pathways described in the nose-to-brain literature, which reach the cranial compartment without that crossing.
For a rapidly degraded heptapeptide this distinction is not academic. Time in circulation is time exposed to plasma peptidases, and the fragment profile above shows how fast that proceeds. A route that shortens the distance to the target tissue is addressing a real constraint of the molecule rather than a preference about needles.
What is still missing
The obvious study has not been published. There is no controlled comparison administering Selank intranasally and parenterally in the same model, measuring brain and plasma exposure for both, and reporting a ratio. The 2006 work established brain pharmacokinetics after intranasal dosing; it did not benchmark that against injection.
The clinical record is also almost entirely Russian-language, and the compound has no regulatory approval outside that context. Trials that exist are small by contemporary standards. Anyone citing Selank's anxiolytic profile is citing a literature that has not been replicated at scale in independent groups.
And the general caution about rodent nasal work applies with full force. Rats carry proportionally far more olfactory epithelium than humans, which is the exact surface the nose-to-brain route depends on. A rodent intranasal result establishes that the pathway operated in that animal, not a human delivery fraction.
The takeaway
Selank is a rare case where the intranasal route carries the published pharmacokinetic work and the injectable route does not. Tritium labelling put it in rat brain tissue after intranasal dosing and mapped its degradation to TKPRP, TKP, RP and GP. The Pro-Gly-Pro tail slows that degradation without stopping it. No head-to-head route comparison exists. We stock Selank as a nasal spray and as a lyophilised vial, and for this compound the spray is the format the pharmacokinetic literature actually used. That is the opposite of the usual situation, and worth recording as such. The closely related Semax was characterised by the same group using the same method.
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References
1. Zolotarev, Yu. A., Dadaian, A. K., Dolotov, O. V., et al. (2006). Evenly tritium-labeled peptides and their in vivo and in vitro biodegradation. Bioorganicheskaia Khimiia, 32(2), 183-191.
2. Agrawal, M., Saraf, S., Saraf, S., et al. (2018). Nose-to-brain drug delivery: An update on clinical challenges and progress towards approval of anti-Alzheimer drugs. Journal of Controlled Release, 281, 139-177. https://doi.org/10.1016/j.jconrel.2018.05.011
3. Md, S., Mustafa, G., Baboota, S., & Ali, J. (2015). Nanoneurotherapeutics approach intended for direct nose to brain delivery. Drug Development and Industrial Pharmacy, 41(12), 1922-1934. https://doi.org/10.3109/03639045.2015.1052081
