Liraglutide: How Scientists Engineered a Long-Lasting GLP-1

Liraglutide's Genesis: How Scientists Engineered a Long-Lasting GLP-1 Breakthrough

Imagine having a powerful key that could unlock better blood sugar control and even help with weight management. That's essentially what our bodies have in 

Glucagon-Like Peptide-1 (GLP-1), a natural hormone produced in our gut. Since its discovery in 1984, GLP-1 has fascinated scientists due to its remarkable benefits: it stimulates insulin release when blood sugar is high, inhibits the hormone glucagon (which raises blood sugar), slows down how quickly food leaves the stomach, and even helps protect pancreatic cells. It was even known to suppress appetite.

With such a promising profile, GLP-1 seemed like an ideal candidate for treating type 2 diabetes. However, there was a major hurdle: native GLP-1 is incredibly fragile and short-lived in the body. It's rapidly broken down by an enzyme called DPP-IV (dipeptidyl peptidase IV) and quickly cleared by the kidneys. Its natural half-life—the time it takes for half of the substance to disappear from the bloodstream—is a mere 1.5 minutes after intravenous (IV) injection and about 1.5 hours after subcutaneous (under-the-skin) injection. This fleeting existence made it practically impossible to use as a daily medication, which typically requires stable blood levels over many hours or even days. Adding to the challenge, GLP-1 itself was difficult to formulate into a stable drug solution.

The Quest for a Longer-Lasting Solution

This is where the groundbreaking research, published in the Journal of Medicinal Chemistry in 2000 by a team from Novo Nordisk, led by Lotte B. Knudsen, comes in. Their ambitious goal was clear: to design "potent derivatives of glucagon-like peptide-1 with pharmacokinetic properties suitable for once daily administration" for patients with type 2 diabetes.

Their ingenious solution centered on a technique called fatty acid derivatization. Think of it like adding a special chemical 'tail' (a fatty acid) to the GLP-1 molecule. The brilliant idea behind this was to make the modified GLP-1 bind to serum albumin. Serum albumin is the most abundant protein in our blood, and it acts like a natural carrier. By hitching a ride on albumin, the GLP-1 derivative is protected from rapid breakdown and kidney clearance, significantly extending its time in the body. This concept of "protraction by facilitating binding to serum albumin" had already shown success in prolonging the action of insulin.

The researchers also explored adding "spacers" (like gamma-L-glutamoyl) between the GLP-1 molecule and the fatty acid tail. These spacers could further enhance the molecule's ability to bind to albumin and improve its solubility in the body.

Engineering the Perfect Molecule: Structure-Activity Relationship

The team undertook a painstaking process of structure-activity relationship (SAR) studies. This involves systematically making many slightly different versions of the GLP-1 molecule, attaching fatty acids of various lengths at different positions, and then testing how these changes affect the molecule's performance. Their goal was to find the optimal combination that offered both high potency and a long-lasting effect.

Here's how they tested their creations:

• Potency (EC50): They measured how strongly each modified GLP-1 activated the human GLP-1 receptor, using a functional assay in laboratory cells. A key measure was the EC50, which stands for "Effective Concentration 50%." In simple terms, it's the concentration of the drug needed to achieve half of its maximum effect. A lower EC50 value means a more potent compound, as less of the drug is needed to get a significant response. Native GLP-1 had an EC50 of 55 pM (picomolar).
• Pharmacokinetics (Half-life): Crucially, they measured the plasma half-lives of their most promising compounds after administering them subcutaneously to pigs. Pigs were used because their physiology is similar enough to humans for these types of studies.

The Breakthrough: Liraglutide Emerges

Their extensive research yielded remarkable results. They discovered that:

• Attaching fatty acids of 12 carbon atoms or longer significantly extended the compound's action.
• They could attach these fatty acids to various positions in the C-terminal (tail end) part of the GLP-1 molecule without losing much potency. However, modifying the N-terminal (head end) generally led to a substantial loss of potency.
• The length of the fatty acid mattered for potency; generally, longer fatty acids could lead to some loss of potency.
• The team successfully identified several compounds that were not only highly potent (with EC50 values comparable to or even better than native GLP-1)      but also had significantly extended half-lives.

Among these successful compounds was what we now know as liraglutide (referred to as compound 5 in the paper). This specific derivative, modified with a gamma-L-glutamoyl-C16 fatty acid chain at Lysine 26, showed excellent potency (EC50 of 61 pM). More importantly, its plasma half-life in pigs was a remarkable 13 hours, a dramatic improvement compared to the native GLP-1's 1.2 hours. Other promising compounds they identified had half-lives ranging from 9 to 31 hours.

This study demonstrated that these engineered GLP-1 derivatives were "suitable for once daily administration to type 2 diabetic patients". 

The Legacy of a Multidisciplinary Effort

The success of this research was a testament to the collaborative, multidisciplinary approach at Novo Nordisk. The authors listed on the paper, including 

Lotte B. Knudsen (who also served as the corresponding author), Per F. Nielsen, Per O. Huusfeldt, Nils L. Johansen, Kjeld Madsen, Freddy Z. Pedersen, Henning Thøgersen, Michael Wilken, and Henrik Agersø, brought together expertise from molecular pharmacology, protein chemistry, medicinal chemistry, assay & cell technology, and pharmacokinetics. Their combined efforts in systematically synthesizing, testing, and characterizing a large series of GLP-1 derivatives were critical to identifying the optimal design.

This pioneering work by Knudsen and her team was a cornerstone in transforming GLP-1 from an intriguing scientific concept into a viable, effective medication. Their discovery of liraglutide's prolonged action paved the way for its clinical development and subsequent approval, first for type 2 diabetes and later for obesity, truly revolutionizing treatment options for these chronic conditions. This paper stands as a monumental contribution to pharmaceutical science and metabolic medicine.