The receptor
The IGF-1 receptor is a receptor tyrosine kinase, structurally similar to the insulin receptor. Activation triggers PI3K/Akt and MAPK cascades, associated with cell growth, survival and proliferation.
Overlap with insulin
The two receptors are similar enough that their ligands cross-react to a degree, and hybrid receptors containing subunits of both exist. This overlap is why IGF-1 research frequently discusses insulin signalling, and why selectivity between the two is a recurring question.
Binding proteins
Most circulating IGF-1 is bound to IGF-binding proteins rather than free. Those proteins control availability, and an analogue with reduced binding-protein affinity behaves differently in a whole system than in a cell culture where the proteins are absent.
This is the single largest reason in vitro and in vivo results for IGF-1 analogues can diverge.
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