Growth hormone secretagogues: what they can and can’t do

This class has better human pharmacology data than most of the grey market. A 2006 study in healthy adults found that a single injection of CJC-1295 produced dose-dependent increases in growth hormone of two- to ten-fold lasting six days or more, and IGF-1 increases of 1.5- to three-fold lasting nine to eleven days, with an estimated half-life of about six to eight days. That is a real, measured, published pharmacodynamic effect.

MK-677 does the same orally. In a two-year randomised, placebo-controlled trial in healthy older adults, it raised growth hormone and IGF-1 into the range of healthy young adults.

So the mechanism is not in dispute. The question is what follows from it.

In that same two-year MK-677 trial, fat-free mass increased by 1.1 kg while the placebo group lost 0.5 kg — and the increase produced no improvement in strength or physical function. Meanwhile fasting glucose rose and insulin sensitivity fell.

Ipamorelin has been tested in a randomised, placebo-controlled trial for postoperative ileus. It was well tolerated, and the median time to first tolerated meal was 25.3 hours on ipamorelin versus 32.6 hours on placebo — a difference that did not reach statistical significance. The trial did not demonstrate benefit.

This is the pattern across the class: the surrogate endpoint moves and the outcome does not. IGF-1 is easy to measure and easy to sell. Strength, function, and recovery are what people actually want, and they have not followed.

A phase IIb trial of MK-677 in 123 elderly patients recovering from hip fracture — precisely the population in whom improved function would matter most — was terminated early after a congestive heart failure safety signal. The investigators concluded the compound had an unfavorable safety profile in that population.

The mechanism is not mysterious. Growth hormone drives fluid retention and reduces insulin sensitivity. Those are not side effects bolted onto the drug; they are consequences of the axis being stimulated. Anyone using this class chronically without periodic bloodwork is choosing not to see the trade.

IGF-1 is also a growth signal, which is the basis for the principal theoretical long-term concern in anyone with an existing or suspected malignancy.

The standard defence of secretagogues over injected growth hormone is that they work through the body’s own axis, preserving pulsatility and negative feedback, so the ceiling stays physiological. That is mechanistically true and it is a genuine advantage.

It does not, however, make the downstream effects of elevated IGF-1 different. The glucose and fluid effects observed in the MK-677 trials arrived through exactly this "natural" pathway.