CJC-1295 and Ipamorelin Half-Life and Pharmacokinetics

Key parameters

All values are from early-phase studies with small samples. These are not population PK estimates from large regulatory dossiers. Individual variability is not well characterized for either compound.

Why the DAC modification changes everything

Native GHRH and the basic modified analog (CJC-1295 without DAC / Mod GRF 1-29) are eliminated rapidly. Peptides of this size are subject to two main clearance mechanisms: enzymatic degradation in plasma (primarily DPP-4 at the N-terminus, plus other proteases) and renal filtration. The four amino-acid substitutions in Mod GRF 1-29 address the first problem by blocking DPP-4 cleavage, but do not address the second. The result is a half-life of roughly 30 minutes.

The DAC (drug affinity complex) strategy adds a maleimidopropionic acid (MPA) group to the C-terminus of the peptide. In plasma, MPA reacts selectively with the free thiol of Cys-34 on serum albumin, forming a stable thioether bond. The resulting albumin-peptide conjugate has the effective size of albumin (66 kDa) rather than the peptide alone (approximately 3.4 kDa). Because albumin is above the glomerular filtration threshold, it largely avoids renal clearance and circulates with a half-life of approximately 19 days in humans.

CJC-1295 does not circulate with albumin's full 19-day half-life, because the peptide is still subject to some proteolysis and the albumin-bound fraction turns over as albumin itself turns over. The observed half-life of approximately 6-8 days in the Phase I/II study reflects the combined kinetics of albumin turnover and residual peptide clearance.^[1]

The albumin-bound peptide can still access the GHRH receptor because albumin extravasates through fenestrated capillaries, including those supplying the anterior pituitary portal circulation. The slow release of the peptide from the albumin-peptide complex over days produces a sustained, low-amplitude GHRH signal rather than the sharp pulsatile stimulus of native GHRH.

CJC-1295 with DAC: what the Phase I/II study reported

Teichman et al. (2006) reported results from a dose-ranging Phase I/II study in healthy adults given a single subcutaneous injection of CJC-1295 with DAC at doses of 30, 60, 90, or 120 mcg/kg, or placebo.^[1] Key findings:

• Mean GH concentrations increased 2- to 10-fold above baseline, with effects sustained for approximately 6 days at all active doses.
• Serum IGF-1 increased 20-60% above baseline, with peak effect at approximately 9-11 days and gradual return toward baseline over 1-2 weeks.
• The estimated mean half-life across doses was approximately 5.8 to 8.1 days (depending on dose group and measurement method).
• Steady-state would be expected to require approximately 4-5 half-lives to reach with repeated weekly dosing.

A subsequent study (Sackmann-Sala et al., 2009) confirmed IGF-1 increases of similar magnitude and used serum protein profiling to characterize the downstream effects of CJC-1295-driven GH-axis activation.^[2] These are pharmacodynamic studies, not efficacy or outcomes studies.

Limitations of this PK evidence: the study enrolled small numbers of participants (exact n by group not repeated here to avoid misquoting from memory), used a single-dose design, and followed subjects for 28 days. Steady-state PK, the effect of repeated dosing on GH-axis feedback, and variability across age, sex, or body-composition categories are not established from this literature.

Ipamorelin pharmacokinetics: what is and is not known

Ipamorelin's half-life of approximately 2 hours in early human studies reflects its small size (pentapeptide, approximately 712 daltons) and lack of a half-life-extending modification. The compound is subject to plasma peptidase degradation and renal filtration. Without a fatty acid or albumin-binding moiety, it clears rapidly.

The principal selectivity finding from Raun et al. (1998) -- that Ipamorelin produces GH release with minimal cortisol or ACTH elevation -- was demonstrated in rats at doses producing substantial GH responses.^[3] Human PK data from the Novo Nordisk development program for post-operative ileus are not fully available in the peer-reviewed literature. The approximate 2-hour half-life figure cited in secondary sources likely originates from internal Novo Nordisk data rather than a published peer-reviewed PK study; this should be considered when interpreting precision of that figure.

There are no published peer-reviewed data on Ipamorelin subcutaneous bioavailability in humans. Whether the bioavailability after subcutaneous injection is comparable to intravenous administration (as used in some animal studies) has not been established in a published human PK study.

PK considerations when combining the two compounds

Because CJC-1295 with DAC and Ipamorelin have substantially different half-lives (approximately 6-8 days vs. 2 hours), they would reach steady state at different rates and produce different plasma-concentration time profiles when dosed together. A commonly described regimen pairs weekly CJC-1295 with DAC with daily or twice-daily Ipamorelin, so that the shorter-acting ghrelin-pathway agonist provides acute GH pulses against the background of sustained GHRH-receptor stimulation from the longer-acting compound.

Whether this combination produces a PK interaction (one compound affecting the clearance or receptor availability of the other) has not been studied. There is no published human PK data for the combination at any dose.

The combination also involves two compounds with different routes of albumin interaction: CJC-1295 with DAC covalently binds to Cys-34 of albumin, which is the same site used by some fatty acids and certain drugs. Whether co-administration of Ipamorelin (which does not bind albumin) affects this interaction is not established.

What is not known

• Bioavailability: subcutaneous bioavailability in humans has not been rigorously published for either compound.
• Steady-state PK: what happens to GH and IGF-1 levels after weeks or months of repeated dosing, particularly whether feedback mechanisms attenuate the GH response, is unknown.
• Special populations: the effect of age, sex, obesity, renal impairment, or hepatic impairment on the PK of either compound has not been published in peer-reviewed literature.
• Drug interactions: no published data exist on interactions between either compound and common medications.
• PK of the combination: no human PK data for the two compounds administered together.