Peptides should make ideal drug molecules as they display high target affinity and selectivity, low inherent toxicity, and are easy to synthesize. Unfortunately, peptides also have a short half-life in the body, partly due to enzyme degradation, but primarily because their small size means that they are filtered out of the blood by the kidneys, usually within minutes. The peptide drug insulin, for example, has a half-life of just 4 to 6 minutes once it reaches the bloodstream. Intravenously administered oxytocin has a half-life of 10 to 15 minutes.
Fast renal clearance means that most approved peptide drugs are designed to act very quickly, and diseases that need longer drug half-lives aren’t well served by peptide candidates, explain Alessandro Zorzi, Christian Heinis, Ph.D., and colleagues at the Institute of Chemical Sciences and Engineering, School of Basic Sciences, Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland.
One potential way of increasing the half-life of peptides in the circulation is to use a ligand to piggy-back the peptide onto longer-lived blood serum proteins, such as albumin. Serum protein albumin is the most abundant blood serum protein and has a half-life of 19 days. Ideally the ligand would be simple to synthesize and attach to the peptide, and also have a high affinity to albumin. Insulin detemir, insulin degludec, and liraglutide are three peptide drugs that contain albumin-binding fatty acids in their structures, and are already used in the clinic.
There have been previous attempts to develop ligands based on either fatty acids or peptides, the EPFL team notes. However, tests with insulin have shown that while fatty acid ligands were found to extend insulin half-life they did not bind albumin very strongly. Conversely, peptide-based ligands bound well to albumin, but demonstrated poor solubility, which affected insulin distribution.
The EPFL researchers conceived a ligand design that would combine the benefits of both peptides and fatty acids by conjugating fatty acid to a short peptide via an amino acid side chain. Ideally the fatty acid alone would bind to albumin with an affinity in the low micromolar range and the peptide moiety would boost that affinity by forming additional contacts to albumin.
Reporting on their work in Nature Communications, the EPFL researchers describe an iterative approach to ligand development, based on rounds of peptide modification and screening, through which they generated a peptide sequence that increased fatty acid binding 27-fold. The resulting highly soluble chimera ligand, or tag, binds human serum albumin with a high affinity (Kd = 39 nM), and can be added on to peptide molecules using standard synthesis techniques In animal models, the new ligand was shown to prolong the half-lives of bioactive peptides 25-fold, to up to 7 hours. In one in vivo evaluation,the ligand increased the half-life of a peptide Factor XIIa inhibitor from about 15 minutes to up to 5.6 hours, which maintained the peptide’s anticoagulation activity in rabbits for up to 8 hours.
Encouragingly, given that the tag has a weaker affinity for rabbit and rat albumin than for human albumin, it’s likely that the half-lives of peptides tagged using the chimeric ligand will be even higher in humans, the team indicates. “This high-affinity albumin ligand could potentially extend the half-life of peptides in human to several days, substantially broadening the application range of peptides as therapeutics,” the authors write in their published paper, which is titled, “Acylated Heptapeptide Binds Albumin with High Affinity and Application as Tag Furnishes Long-Acting Peptides.”
“With this tag in hand, it should be possible to expand the application range of peptide therapeutics from the current mostly short-lived agents that act mainly as receptor agonists to long-acting peptide drugs that can also address targets that require actions over extended time periods,” they conclude.
“We expect that the tag presented in our work will interest a larger research and business audience because it is applicable to virtually any peptide moiety, including small proteins,” professor Heinis commented. “The ligand can be appended to any peptide during solid-phase peptide synthesis on standard synthesizers, making it easily accessible for academic and industry labs.”
Professor Heinis confirmed to GEN that the EPFL team is carrying out animal studies with ligand-tagged therapeutic peptides that are expected “to benefit greatly from a prolonged in vivo half-life.” The identity of the peptides or targets could not be disclosed, however. The ligand could also be used to effectively retrofit existing peptide therapeutics, Professor Heinis suggested. “This is a very attractive application of this tag; we expect that it will be used for this purpose too.”