IGF-1 LR3: Binding Protein Evasion and a Sixteen-Paper Literature

Design and the binding protein rationale

In circulation, the large majority of IGF-1 is bound to IGF binding proteins, principally IGFBP-3 in a ternary complex with the acid-labile subunit. Binding protein association governs half-life and the fraction available to engage the IGF-1 receptor.

The position-3 substitution substantially reduces IGFBP affinity. The stated design intent is a larger free fraction at equivalent total concentration. The N-terminal extension originates in the recombinant expression construct rather than being a deliberate pharmacological feature.

Reduced binding protein affinity is not a uniformly favourable property. Binding proteins also protect the ligand from clearance and provide a reservoir, so evading them changes distribution and duration as well as free fraction. Whether the net effect is desirable depends entirely on the experimental question.

The fetal sheep literature

A striking share of the indexed work uses fetal sheep. Stremming et al. (Am J Physiol Endocrinol Metab, 2021) report that IGF-1 infusion increases organ growth but not by stimulating nutrient transfer to the fetus. White et al. published in the same journal in 2021 on reduced glucose-stimulated insulin secretion following a one-week infusion, and in J Dev Orig Health Dis (2023) reporting that the attenuated secretion does not persist in isolated islets.

White et al. (Am J Physiol Endocrinol Metab, 2025) report that IGF-1 LR3 does not promote growth in late-gestation growth-restricted fetal sheep. This is a negative result in the most-used model for this compound, published recently, and it deserves more weight than its citation count will initially suggest.

Other systems

Engel et al. (J Alzheimers Dis, 2025) report that intranasal Long R3 IGF-1 promotes amyloid plaque remodelling in cerebral cortex but fails to preserve cognitive function in male 5XFAD mice. This is a dissociation between a biomarker and a functional endpoint, and it is a useful cautionary result.

Yavuz et al. (Int J Biol Macromol, 2025) use controlled IGF-1 LR3 release from a nerve conduit for sciatic nerve regeneration in rats, which is a delivery-system study where local concentration rather than systemic exposure is the variable.

Hill et al. (Domest Anim Endocrinol, 1999) on protein metabolism in beef heifers is the oldest indexed work and remains one of the few whole-animal metabolic studies.