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Specific Sweat Test Better Detects Kalydeco’s Effects on CFTR Function, Study Suggests

The effects of CFTR modulator therapies such as Kalydeco (ivacaftor) can be more reliably assessed using a test that specifically measures the sweating dependent on CFTR, called C-sweat rate, than using classic sweat tests or lung function measurements, a study reports.

This kind of test was sensitive enough to reveal that Kalydeco also amplifies CFTR function in healthy individuals, an effect that standard salt measurements in sweat failed to detect.

Hundreds of different mutations in the CFTR gene have been found to cause cystic fibrosis (CF). Some mutations interrupt the production of the CFTR protein, while others disrupt its shape (e.g., F508del, the most common CF mutation) or proper function.

The CFTR protein works as an ion channel across the membrane of cells, and conducts chlorine ions out of cells, regulating the production of mucus, sweat, saliva, tears, and digestive enzymes.

Among the hundreds of CF-causing mutations, there is a group referred to as gating mutations. These are modifications of the CFTR gene that drive the production of an abnormal CFTR channel whose opening, or gate, remains closed, blocking the passage of chloride ions.

Kalydeco, an oral medicine marketed by Vertex Pharmaceuticals, is used to relieve symptoms in CF patients with gating mutations because the therapy works as a CFTR potentiator. It restores channel activity, prolonging the time the gate is open; as a result, the flow of water and salts from cells is more normalized, improving hydration and mucus clearance.

But tests done with cells cultured in the lab have given contradictory results regarding the effects of Kalydeco over normal, non-CF CFTR or in F508del-mutated CFTR, either improving, having no effect, or even reducing the channel’s activity. The underlying mechanism for the action of Kalydeco is not completely understood, adding to the controversy regarding its effects.

Moreover, assessing CFTR function in vivo is challenging, making it difficult to do accurate measurements in human subjects.

Lung function tests, such as forced expiratory volume in one second (FEV1) “are insensitive when lung disease is either too mild or too severe,” according to the researchers, while sweat tests measuring salt (chlorine) concentration in sweat may not reflect CFTR function in organs other than the skin.

Recognizing these issues, the team of researchers investigated the effects of Kalydeco on CFTR in healthy, non-CF individuals, using two complementary assays: sweat chloride levels (a standard for diagnosing CF), and CFTR-dependent sweat rate (C-sweat).

The latter requires the activity of CFTR, which is absent in CF patients and half-normal in carriers of CFTR mutations, “thus providing a near-linear readout” of the channel’s function, according to the study.

As an additional control, the researchers measured sweat production under stimulation by methacholine (M-sweat), which triggers a sweating pathway independent of CFTR.

The study (NCT02310789) enrolled eight non-CF adults, including five without a CFTR mutation and three who carried one F508del mutation.

All were given Kalydeco in an on-and-off regimen — four-day Kalydeco treatment, followed by a 32-day washout, repeated three times. For each subject, more than 100 sweat glands from the arm were analyzed.

Results showed that Kalydeco significantly increased C-sweating, on average by 18%, confirming prior results that showed the therapy could increase CFTR opening even when the ion channel was working properly. This does not apply, however, if CFTR is stimulated to work at its maximum, in which case Kalydeco has no effect.

Consistent with the therapy’s action on CFTR, M-sweat rates — which do not require the activation of the channel — were not changed by Kalydeco treatment.

Importantly, standard sweat chloride tests failed to detect Kalydeco’s effect on CFTR in non-CF patients. This suggested that CFTR function was “most reliably detected with C-sweat rate assays … and not detected with M-sweat measurements or sweat chloride assays,” the team wrote.

Given the results, researchers called attention to the fact that “when a drug that is efficacious in vitro fails to show in vivo effects it is commonly interpreted to mean that the drug has failed, but the present results show that real effects can be masked by features of the in vivo measurements used.”

The team noted that this point applies particularly to the “gold standard” lung function measure of FEV1, “which is strongly influenced by environmental factors and is insensitive to improved CFTR function especially at early or late stages of disease.”

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