Sapropterin trial, part 1: genetics & patience.

Sapropterin trial, part 1: genetics & patience

The UK’s decade-long fight for sapropterin required resilience, determination, and patience. Now that the drug has been approved, our patience is being tested again. A year after I went in for blood tests to determine if my PKU would respond, I finally took my first dose of sapropterin. (For a quick explanation of why the testing is needed, see The genetics of PKU.)

Sapropterin result

After the blood test, I waited as the weeks turned into months. Finally, it was seven months before the phone call. The result was the one which I had expected, but hoped not to receive. My clinic told me that I was unlikely to respond to sapropterin.

This was followed by a letter which confirmed that I would not start trialing sapropterin, and which included the genetics results. One of my two variants was classical, and not responsive to sapropterin. The other was unknown, but deemed unlikely to respond. I did ask my clinic about this, but was told the lab had looked at the unknown variant and determined that it would not respond.

Only 25% of people with PKU are likely to respond to sapropterin, and I had known for years that I was unlikely to be one of them. But, after waiting 42 years for a treatment beyond a restricted diet, it was terribly disappointing. I cried over what would not be, and moved on.

Sapropterin variants

Until the NSPKU conference, where I learned that some patients who had the same result as me (one unknown variant) were still trialling sapropterin to see if the unknown gene was responsive.

At my next PKU appointment, I asked the question. There was a long discussion, during which I learned that they estimated a 2% probability of responding. That is a depressingly small percentage. But it is not zero. And the clinic offered me a sapropterin trial, despite the very low chance of a response.

You might be surprised to learn that I did not accept this immediately. Given the disappointment which the letter had initially provoked, I did not leap in at the news they would allow me to trial sapropterin if I wished. Instead, I took a few days to decide whether I wanted to do this. I knew from other friends and from social media that the trial was difficult. And could I cope with the disappointment if, once again, I was told it wouldn’t respond?

Start of the trial

In the end, I took the trial. I had to. It is the only way to know for sure. And so, it was in the inauspicious month of December, that I started on the baseline testing.

Sapropterin trial, part 2: Baseline testing for Kuvan PKU
Maintaining this level of control would be a tough ask. But I discovered that, when I thought I had been strict on my diet in the past, I really hadn’t.
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