Selank Explained: The Tuftsin-Plus-Pro-Gly-Pro Heptapeptide and What Labs Measured

Selank is easiest to understand as a piece of molecular carpentry. Take tuftsin, a four-residue immunomodulatory peptide, and bolt a Pro-Gly-Pro tail onto its C-terminal end. The result is a seven-residue chain that Moscow research groups have published on since at least 2001. This Vial Post explainer reads the sequence piece by piece, summarizes what the rodent and enzyme papers actually measured, and sets out how FDA treats selank in its compounding review.

Nothing here is guidance on use in people or animals. The Vial Post covers selank only as a subject of laboratory and literature research.

At a Glance

• Sequence: Thr-Lys-Pro-Arg-Pro-Gly-Pro, listed by PubChem under CID 11765600 (PubChem: Selank).
• Formula and mass: PubChem gives C33H57N11O9 and a molecular weight of 751.9 g/mol (PubChem: Selank).
• Other names: PubChem synonyms include TP-7 and the CAS number 129954-34-3 (PubChem: Selank).
• Parent fragment: tuftsin, H-Thr-Lys-Pro-Arg-OH, CID 156080, molecular weight 500.6 g/mol (PubChem: Tuftsin).
• Regulatory note: FDA lists "Selank acetate (TP-7)" among bulk drug substances nominated but withdrawn from category 2 (FDA bulk substances page).

Reading the Sequence

The two halves of selank come from different places:

1. Thr-Lys-Pro-Arg. This is tuftsin. A 2017 review in Current Medicinal Chemistry gathered eighty-six documents on tuftsin and its analogs and grouped the analogs by anti-tumor, anti-inflammatory, antimicrobial and anti-viral activity reported in the literature (Siebert 2017). 2. Pro-Gly-Pro. The added C-terminal tripeptide is the same tail found on semax, a separate heptapeptide built from a fragment of ACTH (Kost 2001; Tabbì 2015). Comparing the two side by side is a useful way to see what changes when only the front half differs.

Subtracting tuftsin's formula from selank's leaves exactly the atoms of three added residues minus the water lost at each new peptide bond, so the PubChem records are internally consistent. The C-terminus stays a free carboxylic acid, as the PubChem IUPAC name ends in "pyrrolidine-2-carboxylic acid".

What the Laboratory Papers Measured

Most of the primary literature is in rats or in isolated tissue and serum. The highlights, described by what was measured rather than by any claimed benefit:

• Enzyme inhibition in serum. Kost and colleagues reported that selank and semax both inhibited enkephalin-degrading enzymes in human serum, with IC50 values of 20 microM for selank and 10 microM for semax. Pentapeptide fragments also inhibited the enzymes; tri-, tetra- and hexapeptide fragments did not (Kost 2001).
• A second enzyme study. The same group reported that selank inhibited enzymatic hydrolysis of plasma enkephalin with an IC50 of 15 microM, and was more potent than the peptidase inhibitors bacitracin and puromycin (Zozulya 2001).
• Gene expression in rat cortex. Volkova and co-workers used real-time PCR on 84 neurotransmission genes in rat frontal cortex. One hour after selank or GABA, 45 genes showed significant expression changes; at three hours, 22 did. The changes after selank correlated with those after GABA (Volkova 2016).
• Hippocampal transcriptome. A cDNA microarray study in rats found mRNA levels of 36 genes changed more than 2-fold after a single intranasal administration and 20 genes after a course; most encoded plasma-membrane-associated proteins (Kolomin 2013).
• Receptor binding. A radioligand study on brain membrane preparations reported that selank affected [3H]GABA binding as a positive allosteric modulator and blocked the modulatory activity of diazepam and olanzapine in that assay. The authors also used HPLC to check reagent and peptide purity (Vyunova 2018).

These are mechanistic and animal observations. They do not establish that selank does anything in people, and The Vial Post does not report administered amounts.

FDA's View

In its page on bulk substances that may present significant safety risks, FDA places selank acetate (TP-7) in the table of nominations withdrawn from category 2. The agency says compounded drugs containing it may pose a risk of immunogenicity for certain routes of administration because of the potential for aggregation and peptide-related impurities. It also says it lacks important information about safety issues raised by selank acetate administered to humans (FDA bulk substances page).

Our report on the interim compounding policy covers how that list fits into the wider review.

Reader Questions

Is selank a tuftsin analog? Yes. Its first four residues, Thr-Lys-Pro-Arg, are the tuftsin sequence, followed by Pro-Gly-Pro (PubChem: Selank; PubChem: Tuftsin).

How is selank related to semax? Both are synthetic heptapeptides ending in Pro-Gly-Pro, and both were tested in the same serum enzyme assay (Kost 2001). See our semax explainer.

What molecular weight should a COA show? PubChem lists 751.9 g/mol for the free peptide (PubChem: Selank). Material sold as an acetate salt carries counter-ions, so read the certificate for which form was measured.

Lab Note

Because selank is short and proline-rich, identity is easy to confirm by mass spectrometry and purity by HPLC. Our explainers on HPLC versus mass spec and reading a Certificate of Analysis walk through both.

For research and educational use only. This article is not medical advice and does not describe dosing or human use.