IGF-1 LR3 vs IGF-1 DES: Structural and Pharmacokinetic Differences

IGF-1 LR3 and IGF-1 DES are the two IGF-1 analogues you see most often on a lab bench, and the only reason either one exists is binding protein interference. Native IGF-1 spends almost all of its time in circulation stuck to IGF binding proteins, which makes concentration-response work messy. Both analogues were engineered to dodge that problem, but they do it in different ways, and the consequences for potency, clearance and experimental design are not the same. Here is what the primary literature from the Adelaide groups who built these molecules actually measured.

Same problem, two solutions

Start with the nuisance these molecules were designed around. Cells in culture secrete IGF binding proteins into the medium, and those proteins sequester IGF-1 before it reaches the type 1 receptor. Ross and colleagues showed this cleanly in 1989: a binding protein purified from bovine kidney cells knocked down the ability of IGF-2 and IGF-1 to drive DNA synthesis and protein accumulation in chick embryo fibroblasts, but it did essentially nothing to DES IGF-1 [5]. The potencies of the three peptides tracked inversely with how much binding protein they picked up. That is the whole story in one experiment.

DES IGF-1, written properly as des(1-3)IGF-I, is the natural answer. It is a truncated variant with the N-terminal tripeptide Gly-Pro-Glu removed, and it has been isolated from bovine colostrum, human brain and porcine uterus, so it is probably a post-translational cleavage product rather than a purely synthetic idea [1]. Ballard’s review puts its potency at roughly tenfold over IGF-1 for hypertrophy and proliferation in cultured cells, and attributes that squarely to the missing glutamate at position 3 [1].

LR3 is the engineered answer. Francis and colleagues expressed a family of IGF-1 fusion peptides in E. coli, each carrying the first 11 amino acids of methionyl porcine growth hormone ahead of the IGF-1 sequence, then swapped Glu3 for Gly or Arg [2]. Long [Arg3]-IGF-I, which everyone now calls LR3, came out of that series. The hydrophobic N-terminal extension also helped the peptide fold correctly during production, which is a practical detail worth knowing if you ever wonder why the extension is there at all.

How they rank against each other

In cell lines that secrete binding proteins into the medium, the order Francis reported was: LR3 IGF-I and DES IGF-I at the top and roughly level with one another, then Long [Gly3]-IGF-I, then Long IGF-I, then native IGF-1 last [2]. Two structural tricks, one convergent outcome.

Flip the system and the ranking collapses. Chicken embryo fibroblasts put out no detectable binding proteins, and in that medium LR3 came in below IGF-1 [2]. The reason is simple enough: LR3 binds the type 1 IGF receptor about three times more weakly than IGF-1 does [6]. Remove the binding protein handicap and that receptor deficit is all that is left. If your model has little or no binding protein in play, reaching for LR3 buys you nothing and may cost you signal.

Clearance and distribution

The old copy on this topic used to quote tidy half-life numbers for both analogues. We deleted them, because we could not find a primary source for any of them. What we can source is clearance, and the figures are informative on their own.

Ballard’s group tracked iodinated IGF-1, IGF-2 and DES IGF-1 after intravenous delivery in rats [4]. Most of the DES tracer stayed free rather than forming complexes. Total plasma clearances were 4.59, 1.20 and 1.34 ml/min per kg for DES IGF-1, IGF-1 and IGF-2. Steady state volume of distribution was also much larger for DES, 461 ml/kg against 167 and 181. Radioactivity in adrenals, brain, skin, stomach, duodenum, ileum, jejunum and colon ranked DES above IGF-1 above IGF-2, which is the exact reverse of how well the three peptides form complexes with plasma binding proteins.

LR3 behaves the same way and the 1993 clearance comparison nailed the mechanism [3]. LR3 disappeared from plasma much faster than IGF-1. Then the group repeated the comparison in pregnant rats, where binding protein levels are markedly reduced, and IGF-1 clearance accelerated to nearly the LR3 rate. Change the binding protein environment and the difference between the two peptides largely evaporates. That is about as direct a demonstration of causality as rodent pharmacokinetics gets.

What the in vivo studies measured

Keep these as what they are: specific experiments with specific endpoints, mostly in rats and guinea pigs, mostly from one research network in the 1990s.

• Normal female rats given the peptides continuously for 14 days: LR3 came out 6 fold more potent than IGF-1 on body weight gain, nitrogen retention and feed conversion efficiency [3].
• Rats made catabolic with 20 micrograms per day of dexamethasone: both DES IGF-1 and LR3 IGF-1 were about 2.5 fold more potent than IGF-1, and the analogues raised gut weight by up to 45 percent [6]. The authors flagged a paradox worth remembering, that LR3 outperformed despite binding the type 1 receptor less well.
• Guinea pigs infused for 7 days: LR3 at 120 micrograms per day significantly increased the fractional weights of adrenals, gut, kidneys and spleen, while overall growth was not stimulated, and neither IGF-1 nor IGF-2 moved most measures [7].
• Cultured rat anterior pituitary cells: DES IGF-1 maximally stimulated GH secretion at 0.1 micrograms per litre against 1 for IGF-1, and its IC50 for inhibition was about 1 microgram per litre against roughly 20 for IGF-1 [8]. Pituitary cells secrete their own binding proteins, which bound DES less well, so the same mechanism applies at the tissue level.

One pattern runs through all of it. Delivery schedule and binding protein environment changed the result as much as the molecule did. Any protocol comparing these two analogues without matching those conditions is comparing several things at once.

Handling and characterization

Both analogues are small proteins, not short synthetic peptides, and that changes what you should check. Identity by mass spectrometry and purity by reversed phase HPLC are the baseline, but neither tells you whether the material folded correctly, and a misfolded IGF analogue can still show the right mass. Francis noted that the N-terminal extension in the Long series existed partly because it improved folding yields, which is a reminder that correct disulfide pairing is the hard part of making these molecules [2]. If you are sourcing either one, ask what the third party testing covers, and read the certificates of analysis rather than a purity number on a label. Our broader notes on IGF-1 LR3 go into the molecule on its own.

Frequently asked questions

Is IGF-1 LR3 stronger than IGF-1 DES?

Not in any consistent way. In binding protein rich systems both land in the same 5 to 10 fold band above IGF-1 and Francis ranked them together at the top [2][3]. In the dexamethasone rat model both came out about 2.5 fold above IGF-1 [6]. The honest answer is that nobody has published a clean head to head potency ratio across models.

Why does removing three amino acids change so much?

Because one of the three is Glu3, and that glutamate is a contact point for IGF binding proteins [1]. Delete it, or replace it with arginine as LR3 does, and binding protein affinity falls off a cliff while receptor binding survives mostly intact.

Does the N-terminal extension on LR3 do anything besides add length?

It helped the reduced and denatured fusion peptide refold into biologically active product at high yield during E. coli expression [2]. It is a production feature as much as a pharmacological one.

References

1. Ballard FJ, Wallace JC, Francis GL, Read LC, Tomas FM. Des(1-3)IGF-I: a truncated form of insulin-like growth factor-I. Int J Biochem Cell Biol. 1996. PMID 8930132
2. Francis GL, Ross M, Ballard FJ, et al. Novel recombinant fusion protein analogues of insulin-like growth factor (IGF)-I indicate the relative importance of IGF-binding protein and receptor binding for enhanced biological potency. J Mol Endocrinol. 1992. PMID 1378742
3. Ballard FJ, Walton PE, Bastian S, Tomas FM, Wallace JC, Francis GL. Effects of interactions between IGFBPs and IGFs on the plasma clearance and in vivo biological activities of IGFs and IGF analogs. Growth Regul. 1993. PMID 7683526
4. Ballard FJ, Knowles SE, Walton PE, et al. Plasma clearance and tissue distribution of labelled insulin-like growth factor-I (IGF-I), IGF-II and des(1-3)IGF-I in rats. J Endocrinol. 1991. PMID 2005410
5. Ross M, Francis GL, Szabo L, Wallace JC, Ballard FJ. Insulin-like growth factor (IGF)-binding proteins inhibit the biological activities of IGF-1 and IGF-2 but not des-(1-3)-IGF-1. Biochem J. 1989. PMID 2539101
6. Tomas FM, Knowles SE, Owens PC, et al. Insulin-like growth factor-I (IGF-I) and especially IGF-I variants are anabolic in dexamethasone-treated rats. Biochem J. 1992. PMID 1371669
7. Conlon MA, Tomas FM, Owens PC, Wallace JC, Howarth GS, Ballard FJ. Long R3 insulin-like growth factor-I (IGF-I) infusion stimulates organ growth but reduces plasma IGF-I, IGF-II and IGF binding protein concentrations in the guinea pig. J Endocrinol. 1995. PMID 7561636
8. Simes JM, Wallace JC, Walton PE. The effects of IGF-I, IGF-II and des(1-3)IGF-I on growth hormone and IGF-binding protein secretion from cultured rat anterior pituitary cells. J Endocrinol. 1991. PMID 1715381