Thymalin

Thymalin, also spelled timalin or thymaline, is not a single defined molecule. Primary papers describe it as a polypeptide preparation. It is made from calf thymus by mild acid extraction. Its creators called it a complex of peptides, not one compound (Morozov and Khavinson, 1997). Two short peptides are tied to that extract. The dipeptide Glu-Trp was isolated from it and, made by synthesis, became thymogen (Anisimov et al., 2000).

The dipeptide Lys-Glu (vilon) was made by synthesis (Morozov and Khavinson, 1997), and a later review from the group describes it as part of thymalin (Khavinson et al., 2021). It gives no sequence, formula, molecular weight or CAS number. That is consistent with a mixture.

The PubChem name clash matters. CID 3085284 is the nine-residue peptide pGlu-Ala-Lys-Ser-Gln-Gly-Gly-Ser-Asn. Its sequence was worked out from pig serum. It was named facteur thymique sérique, later thymulin (Pleau et al., 1977). Its activity in those assays needed a bound zinc ion (Dardenne et al., 1985; Bach and Dardenne, 1989). "Thymalin" appears there only as a synonym. The extract in the thymalin papers is not that nonapeptide.

Published work falls into four areas. Each summary below states the model used. Two caveats apply to the whole field. First, the work is older and more concentrated than its size suggests. A PubMed search for the name variants returns about 300 records. Roughly 101 list V. Kh. Khavinson or V. G. Morozov as an author.

About 268 are in Russian or Ukrainian, with only 31 in English. More than three quarters date from the 1980s and 1990s. Second, the material is a preparation, not a molecule. So effects credited to "thymalin" in one paper may not refer to the same mixture as in another. Several studies also test it alongside thymogen or vilon.

What the preparation is and what came out of it

The founding group described thymic peptides extracted from calf thymus. They also described how the dipeptide Glu-Trp was separated from that extract by reversed-phase chromatography. The thymogen product was based on that dipeptide. Both the extract and the synthetic dipeptides were reported to affect T-cell differentiation. They were also reported to affect cyclic nucleotide composition in blood lymphocytes.

But their effects on antioxidant responses in thymocytes differed (Morozov and Khavinson, 1997; characterization also in Safronov et al., 1984). An independent review of the period places it in a crowded field. That field included thymosins and thymulin. It also included thymopoietin and thymopentin. The review notes that these are hard to compare (Arion et al., 1997).

Rodent and explant models

Several 1980s studies looked at colony-forming units from bone marrow, spleen and blood. They used mice whose thymus had been removed, plus sham-operated mice. They reported stimulation of granulocytic colonies from splenic colony-forming units. This was seen in both animals and culture. The effect on bone marrow colony-forming units showed up only in animals (Val'kovich et al., 1986; Popov and Ergakova, 1985).

Work from the same period reported less platelet aggregation in intact rats. Aortic antiaggregation activity did not change (Kuznik et al., 1986). Organ-culture work from the Pavlov Institute compared the extract with synthetic peptides derived from it. It used explants of rat thymus and spleen.

The extract boosted explant growth at about 5 ng/mL in tissue from 21-day-old rats. Vilon and thymogen inhibited growth in explants from 1-day-old animals (Chalisova et al., 1999). A companion paper compared five tissue extracts, each on explants of its own source organ. The thymus extract was active at the lowest concentration of the set (Chalisova et al., 2000).

A separate line of work used antibodies raised against the extract. These found immunoreactive material in human fetal reticuloepithelium, epidermis and developing respiratory organs. That epidermal layer thinned by about age 70 (Khlystova et al., 2002; Khlystova et al., 2003). Those are findings about an antibody target in tissue sections, not about the product.

Recent cell-culture work, including independent groups

One study used cultures of human hematopoietic stem cells. The extract was reported to lower CD44 and CD117 expression by two to three times. It was also reported to raise CD28 by 6.8 times. The authors read this as a shift along the differentiation pathway (Khavinson et al., 2020). An Italian group worked with the founding author.

They tested five of these peptides, including the thymalin complex, in the human THP-1 monocyte line. They reported more tyrosine phosphorylation of mitogen-activated kinases. Stimulated cells showed lower tumor necrosis factor and interleukin-6 expression. Adhesion to activated endothelial monolayers fell (Avolio et al., 2022). A 2023 study paired two methods.

It used molecular docking of the Lys-Glu and Glu-Trp dipeptides against double-stranded DNA. It also ran an in vitro inflammation model in blood mononuclear cells from four donors. In stimulated cells, thymalin lowered tumor necrosis factor alpha release to about 45 percent and interleukin-1 beta to about 38 percent of the stimulated control, while its interleukin-6 change was not significant (Linkova et al., 2023).

Three independent lines of work exist. One found fewer cysticerci in a mouse Taenia crassiceps model (Hermánek and Prokopic, 1989). Another is a Vladivostok comparison with a squid-derived peptide preparation (Kuznetsova et al., 2013). The third is rat mandible bone-defect models from Ukrainian groups (Boiko et al., 2024).