Selank: Molecular Structure

Molecular Structure and Properties#

Selank is a peptide whose molecular structure and properties have been characterized through analytical chemistry and structural biology studies.

Amino Acid Sequence#

Selank is a synthetic heptapeptide derived from the endogenous tetrapeptide tuftsin. Its primary structure is Thr–Lys–Pro–Arg–Pro–Gly–Pro (TKPRPGP). Multiple studies explicitly state this sequence and describe Selank as a tuftsin derivative extended at the C-terminus by a Pro–Gly–Pro (PGP) motif that improves stability against proteolysis and prolongs action.

Amino acid sequence and molecular architecture – Sequence: Thr–Lys–Pro–Arg–Pro–Gly–Pro (1-letter: TKPRPGP). The molecule contains the tuftsin core (Thr–Lys–Pro–Arg) followed by a C-terminal PGP extension. – Common modifications: In the gathered sources, Selank is discussed as the unmodified heptapeptide (free N- and C-termini). No terminal acetylation or amidation is reported in the cited excerpts. – Structural features: The PGP motif is a glyproline element associated with protease resistance; primary degradation often starts by eliminating the C-terminal Pro–Gly–Pro, yielding shorter fragments. Activity mapping indicates that C-terminal Gly/Pro residues are important; fragments such as Arg–Pro–Gly–Pro retain biological effects (e.g., modulation of GABA binding sites).

Physicochemical properties (theoretical, based on TKPRPGP) – Average molecular weight: approximately 751.9 Da, calculated from the confirmed sequence as the sum of residue masses minus 6 waters. Direct literature MW was not reported in the retrieved evidence. – Isoelectric point (pI): high, estimated around 11.5–12, because the peptide contains one Lys and one Arg and no acidic side chains. This value is theoretical, as explicit experimental pI was not provided in the retrieved sources. – Charge distribution and net charge by pH: Side-chain pKa values for Lys (~10.5) and Arg (~12.5) dominate the charge profile. With a free N-terminus (pKa ~8.0) and C-terminus (pKa ~3.1), the peptide is predicted to be strongly cationic at neutral pH. Approximate net charges: pH 2 ≈ +3, pH 7.4 ≈ +2, pH 11 ≈ 0 (the transition to neutrality occurs between Lys and Arg deprotonation events). Degradation studies and design rationale emphasize the protease-resistant C-terminal PGP but do not alter these ionizable groups.

Additional structural context – Relation to tuftsin: Selank is a rational extension of tuftsin (Thr–Lys–Pro–Arg), a fragment of human IgG with immunomodulatory activity. The PGP extension was introduced to enhance stability and modulate activity; both Selank and tuftsin are subject to proteolysis, but Selank’s degradation often begins by C-terminal PGP loss.

Embedded data summary

Limitations and notes – The retrieved evidence clearly establishes sequence and structural relationship to tuftsin and the role of the PGP motif. However, we did not find explicit experimental reports of molecular weight or isoelectric point in the provided texts; the MW and pI above are theoretical estimates derived from the confirmed sequence and standard pKa values.

Stability and Formulation#

Selank (Thr–Lys–Pro–Arg–Pro–Gly–Pro) is a short proline-rich “glyproline” heptapeptide whose stability profile is dominated by rapid enzymatic cleavage in biological media. Across the available literature, explicit studies quantifying pH-dependent stability or storage temperature sensitivity (lyophilized versus solution) are lacking; however, multiple sources characterize its enzymatic degradation pathways, identify fragments, and provide limited kinetic observations. Key findings and gaps are summarized below, followed by practical considerations for formulation.

pH stability

• No direct, Selank-specific pH stability studies in simple buffers or nasal spray vehicles were located in the retrieved sources. Reviews and experimental reports focus on metabolism in biological matrices rather than chemical stability across pH ranges, so pH optima or degradation rate constants as a function of pH remain unreported in these texts.

Temperature sensitivity and storage form

• The retrieved primary literature did not report Selank’s stability as a function of storage temperature or compare lyophilized versus solution shelf-life. No validated storage recommendations (e.g., room temperature vs 2–8 C vs frozen) or accelerated-degradation data were found in these sources.

Degradation pathways and kinetics in biological media

• Enzymatic degradation is rapid in plasma and tissues. In vitro assays with evenly tritium-labeled Selank demonstrate a steep decline of the parent peptide over minutes in rat plasma, with dipeptidyl carboxypeptidases accounting for the majority of cleavage; dipeptidyl aminopeptidases and aminopeptidases contribute to a lesser extent. The prominent fragments include the pentapeptide TKPRP, the tripeptide TKP, and the dipeptides RP and GP; additional species observed chromatographically include PRPGP, TKPR, TKPRPG, RPGP, and KPRPGP.
• A plasma half-life on the order of about 2 minutes has been reported, consistent with rapid enzymatic clearance in biological media. Time-course data similarly show substantial parent loss within minutes.
• Intranasal administration leads to relatively higher short-term concentrations in olfactory bulbs than in blood, supporting nose-to-brain exposure despite overall rapid systemic degradation; among fragments, the PGP tripeptide is particularly stable and can persist for hours in plasma.
• Mechanistic note: Selank inhibits enkephalin-degrading enzymes in human serum, which may influence peptide–enzyme dynamics in blood but does not imply long systemic stability of Selank itself.

Formulation considerations

• Because explicit Selank pH/temperature stability data are not reported in the retrieved sources, practical recommendations must be inferred from its rapid enzymatic degradation profile and general peptide-formulation practice. Rapid proteolysis in plasma and tissue homogenates suggests that formulations for local delivery (e.g., intranasal for CNS targeting) can leverage anatomical barriers and exposure kinetics to favor brain delivery while minimizing systemic degradation, consistent with observed olfactory bulb enrichment after intranasal dosing.
• In the absence of Selank-specific buffer/excipient data, general peptide stabilization approaches are reasonable to consider: lyophilized dosage forms with cryo/lyoprotectants (e.g., sugars) and enzyme-inhibiting strategies or protective delivery systems. However, these remain extrapolations; no source provided Selank-specific buffer pH ranges, preservative choices, or packaging constraints.

Conclusion

• What is known with evidence: Selank undergoes rapid enzymatic degradation primarily via dipeptidyl carboxypeptidases, yielding characteristic fragments (TKPRP, TKP, RP, GP and others), with an in vitro plasma half-life of roughly 2 minutes; intranasal dosing produces higher early olfactory bulb levels relative to blood, and the PGP fragment is notably stable in plasma.
• What remains unreported in these sources: quantitative pH stability, temperature sensitivity, and Selank-specific excipient/preservative/packaging recommendations. Consequently, formulation guidance must be inferred from general peptide principles and the observed rapid enzymatic degradation in biological media.

Pharmacokinetics#

We synthesized available pharmacokinetic evidence for Selank (Thr‑Lys‑Pro‑Arg‑Pro‑Gly‑Pro) from preclinical radiolabel studies and reviews. Quantitative parameters exist primarily in rats; formal human PK has not been reported. Where possible, we provide numeric values and experimental context, and note gaps.

Absorption

• Route and rate: After intranasal (i.n.) administration in rats, Selank appears rapidly in plasma and brain. A review summarizing rat data reports detection in plasma by ~30 s and in brain by ~2 min post‑dose, indicating fast systemic uptake and central exposure via the i.n. route. Radiolabel distribution studies show measurable blood levels within 1 min after both i.n. and intraperitoneal (i.p.) administration; representative values include blood radioactivity of ~0.08%/g at 1 min after i.n. versus ~0.24%/g after i.p., consistent with lower systemic exposure but efficient CNS delivery after i.n. dosing. Peak tissue levels across abdominal organs generally occur within 3–5 min after dosing.

Distribution

• Brain penetration and regional levels: Tritium‑labeled Selank given i.n. to rats yields higher early concentrations in several brain regions than in blood. At 3 min post‑i.n. dose, intact Selank measured by HPLC‑radioactivity was: blood 5.92 pM; olfactory bulbs 56 pM; midbrain 11.6 pM; cerebral cortex 3.1 pM; cerebellum 6.1 pM, corresponding to brain:blood ratios of ≈9.5 (olfactory bulb), 2.0 (midbrain), 0.52 (cortex), and 1.03 (cerebellum). Reviews further note comparable brain and blood levels after i.n. dosing, slower decline in brain than blood/abdominal organs, and preferential accumulation in archicortex and diencephalon. Comparative biodistribution tables show that after i.n. dosing brain levels are comparable to or higher than blood and decline more slowly than in peripheral tissues.

Metabolism

• Enzymes and fragments: In rat plasma in vitro, Selank undergoes rapid proteolysis predominantly by dipeptidyl carboxypeptidases (major share of activity), with minor contributions from dipeptidyl aminopeptidases; terminal amino‑/carboxypeptidases contribute little. Primary fragments include TKPRP (pentapeptide), TKP (tripeptide), RP and GP (dipeptides), with further degradation to amino acids; Gly‑Pro shows relative stability in plasma. Mechanistic summaries concur that dipeptidyl carboxypeptidases drive initial cleavage and that metabolites may retain activity.

Elimination

• Time course and unknowns: Intact peptide in blood declines within minutes after both i.p. and i.n. dosing, with >33% decline by 7 min after i.p. and an approximately halved level at early post‑i.n. sampling, while brain levels decline more slowly (qualitative). Classical elimination routes (renal/hepatic), systemic clearance, and volume of distribution were not quantified in the available studies.

Half‑life

• In vitro: In rat plasma in vitro, intact Selank decays with a very short half‑life. HPLC‑radioactivity data show parent peptide dropping from ~895 (arbitrary units) at 0.33 min to ~410 at 2 min and ~8 at 23 min; these data imply an in vitro plasma half‑life on the order of ~1–2 min. A separate preclinical comparison also cites an in vitro Selank half‑life of about 2 min.
• In vivo: Specific plasma or brain half‑life values were not reported. Reviews describe that brain levels after i.n. dosing remain relatively stable and decline more slowly than blood, with pharmacodynamic effects lasting 20–24 h; this duration reflects pharmacodynamics, not plasma t1/2.

Bioavailability

• Intranasal: A review summarizing intranasal delivery studies reports a high “bioavailability” for active substance of 92.8% via the i.n. route, alongside lower systemic concentrations versus i.p., consistent with efficient nose‑to‑brain delivery. Formal absolute bioavailability determined by standard PK methods was not found; the 92.8% figure derives from preclinical distribution/retention assessments rather than human PK.

Key quantitative points and limitations

• Time to detection (rats): plasma ~30 s (i.n.), brain ~2 min (i.n.). Early blood levels at 1 min: i.n. ~0.08%/g vs i.p. ~0.24%/g. Early brain:blood ratios at 3 min (i.n.): olfactory bulb ≈9.5, midbrain ≈2.0, cortex ≈0.52, cerebellum ≈1.03.
• Half‑life: rat plasma in vitro t1/2 ~1–2 min; no reliable in vivo plasma or brain t1/2 reported.
• Bioavailability: intranasal “active substance” availability reported as 92.8% in preclinical literature; no human absolute bioavailability.
• Gaps: No human PK modelling, clearance, or volume of distribution; elimination pathways not directly quantified.

Embedded summary table with values and sources:

Related Reading#

• Selank overview and research guide
• Peptides similar to Selank
• Selank research evidence