Trikafta Mechanism of Action: CFTR Modulators at Work

Cystic fibrosis (CF) is a genetic disorder that affects multiple organs, primarily the lungs and digestive system, due to defective ion transport. This leads to thick mucus buildup, chronic infections, and progressive lung damage. For decades, treatments focused on managing symptoms rather than addressing the underlying cause.

Trikafta represents a major advancement by directly targeting the malfunctioning CFTR protein. The combination of three modulators—elexacaftor, tezacaftor, and ivacaftor—improves CFTR function more effectively than previous therapies. Understanding how each component works provides insight into why this treatment has significantly improved outcomes for many people with CF.

CFTR Protein Basics

The cystic fibrosis transmembrane conductance regulator (CFTR) protein is an anion channel that maintains the balance of salt and water across epithelial surfaces. It is primarily expressed in the epithelial cells lining the lungs, pancreas, intestines, and sweat glands, facilitating chloride and bicarbonate ion transport. This ion movement ensures proper hydration of mucus and other secretions, preventing them from becoming excessively thick and sticky.

CFTR is part of the ATP-binding cassette (ABC) transporter family, regulated by ATP hydrolysis and phosphorylation by protein kinase A (PKA). Structurally, it consists of two membrane-spanning domains (MSDs), two nucleotide-binding domains (NBDs), and a regulatory (R) domain. The MSDs form the chloride channel, while the NBDs control gating by binding and hydrolyzing ATP. The R domain, unique to CFTR, modulates channel activity through phosphorylation.

When CFTR is activated, ATP binding at the NBDs induces conformational changes that open the channel, allowing chloride ions to flow down their electrochemical gradient. This ion movement drives osmotic water transport, ensuring mucus remains hydrated to facilitate mucociliary clearance in the lungs and proper enzymatic function in the digestive tract.

CFTR dysfunction disrupts this process, leading to dehydration of epithelial surfaces and thick mucus accumulation. In the lungs, this impairs mucociliary clearance, fostering bacterial colonization and chronic infections. In the pancreas, defective CFTR impairs bicarbonate secretion, causing enzyme insufficiency and malabsorption. The severity of these effects depends on the extent of CFTR impairment, which varies based on the specific mutation.

CFTR Mutations

Mutations in the CFTR gene disrupt protein function, leading to defective ion transport. Over 2,000 mutations exist, varying widely in their effects. Some prevent CFTR synthesis, while others produce a misfolded protein that is degraded prematurely or fails to function properly at the membrane. These mutations are classified into six groups based on their impact on CFTR production, processing, and function.

Class I mutations, such as G542X, introduce premature stop codons that prevent functional protein production. Class II mutations, including the most common F508del, cause CFTR misfolding and retention in the endoplasmic reticulum, where it is degraded before reaching the membrane. Class III mutations, such as G551D, produce CFTR proteins that reach the membrane but fail to open properly in response to ATP and phosphorylation.

Other mutations impair CFTR function differently. Class IV mutations, including R117H, reduce the channel’s ability to conduct chloride ions, leading to milder disease. Class V mutations, such as 3849+10kbC→T, lower CFTR protein production but do not affect function directly, allowing partial chloride transport. Class VI mutations, like Q1412X, cause CFTR proteins to degrade too quickly after reaching the membrane, limiting their effectiveness. The severity of symptoms depends on the specific mutation combination inherited from both parents.

Elexacaftor as a CFTR Corrector

Elexacaftor improves CFTR function by addressing misfolding and degradation, particularly in F508del mutations. Many CFTR mutations cause the protein to misfold during synthesis, leading to premature degradation before it reaches the membrane. Elexacaftor stabilizes CFTR during maturation, allowing more of it to evade cellular quality control mechanisms and reach the epithelial surface for chloride transport.

By binding directly to CFTR, elexacaftor promotes a stable conformation, reducing recognition by degradation pathways. This enhances CFTR trafficking to the membrane rather than being targeted for proteasomal degradation. Unlike earlier correctors, elexacaftor complements tezacaftor’s actions, addressing distinct structural defects in CFTR folding and processing.

Clinical trials have shown that elexacaftor significantly improves lung function, measured by percent predicted forced expiratory volume in one second (ppFEV1). In patients with at least one F508del mutation, adding elexacaftor resulted in a mean increase of approximately 14 percentage points in ppFEV1 compared to baseline, a significant improvement over previous CFTR modulators. This increase in lung function is accompanied by reductions in sweat chloride levels, a biomarker of CFTR activity, and improvements in body mass index (BMI), reflecting better overall health.

Tezacaftor as a CFTR Corrector

Tezacaftor addresses defective CFTR folding and trafficking, particularly in F508del mutations. Misfolded CFTR proteins are often degraded before reaching the membrane. Tezacaftor stabilizes CFTR during maturation, allowing more of it to escape degradation and reach the epithelial surface, increasing the number of functional chloride channels.

Unlike earlier correctors, tezacaftor has improved pharmacokinetic properties, reducing adverse effects while maintaining efficacy. It was developed to be structurally distinct from previous modulators, enhancing stability and lowering the risk of drug interactions. Studies show that tezacaftor improves CFTR processing efficiency and chloride transport, though its full benefit is realized in combination therapy.

Ivacaftor as a CFTR Potentiator

While correctors like elexacaftor and tezacaftor enhance CFTR trafficking, they do not address channel gating defects. Ivacaftor is a potentiator that improves CFTR function at the cell surface by increasing the probability that the channel remains open, allowing greater chloride ion flow and improved epithelial hydration.

Ivacaftor binds directly to CFTR and stabilizes its open conformation. This is particularly beneficial for gating mutations like G551D, where CFTR reaches the membrane but fails to open properly. Clinical studies show that ivacaftor significantly enhances chloride transport, as evidenced by reductions in sweat chloride levels and improvements in lung function. In individuals with G551D, ivacaftor treatment led to an average increase of 10 percentage points in ppFEV1, along with weight gain and fewer pulmonary exacerbations.

Beyond gating mutations, ivacaftor enhances the small amount of CFTR that reaches the membrane in F508del mutations. By keeping these channels open longer, ivacaftor maximizes chloride transport, contributing to Trikafta’s overall efficacy. The combination of potentiation and correction results in better clinical outcomes than either approach alone.

Synergistic Mechanisms in the Combination

The combination of elexacaftor, tezacaftor, and ivacaftor in Trikafta addresses multiple CFTR defects simultaneously. Each component plays a distinct role, but their combined effect is greater than the sum of their individual contributions.

Elexacaftor and tezacaftor stabilize CFTR during maturation, binding to different regions of the protein to reduce misfolding more effectively than either corrector alone. This dual-corrector approach increases the amount of CFTR reaching the membrane. Once CFTR is present at the cell surface, ivacaftor enhances its activity by keeping the channel open longer, ensuring maximum ion transport.

Clinical trials demonstrate that this combination significantly improves lung function, with mean increases in ppFEV1 exceeding 14 percentage points in patients with at least one F508del mutation. These findings highlight the power of targeting multiple CFTR defects simultaneously, leading to substantial gains in respiratory health and overall well-being.