The Hold on The PheNIX Gene Therapy for Phenylketonuria Has Been Lifted

The FDA has just announced that they have lifted the temporary hold placed on pheNIX gene therapy. This gene therapy was created by Homology Medicine as an investigational treatment for phenylketonuria. The lift of the hold means that research on this gene therapy in this rare patient population may be continued.


Phenylketonuria (PKU) is caused by a deficiency in the enzyme called phenylalanine hydroxylase (PAH). Without PAH, Phe is not metabolized properly. Phe is an amino acid which is present in most protein-rich foods. Without PAH, Phe builds up within the blood. The buildup of Phe is toxic in the brain and can lead to seizures, intellectual disability, tremors, psychiatric concerns, and behavioral issues.

This rare condition is genetic in nature. With the rise of newborn screening efforts in the 1960s-70s, most patients are diagnosed at birth. This quick time to diagnosis leads to faster treatment.

The Hold

The hold on this investigational gene therapy was due to an adverse event observed during a clinical trial. The adverse event was elevated liver function tests. Thankfully, all of the cases were resolved, and no participants necessitated hospitalization.

Homology enhanced risk mitigation by changing the protocol. They also added an immunosuppressive regimen and reduced the duration of the prophylactic steroids. Homology reports that this is the same approach they are using in the pheEDIT trial for PKU, as well as the juMPStart investigation for Hunter syndrome. The goal is to reduce the immune response to the AAVs.

Homology will provide an update on the pheNIX trial in the fall.

You can read more on the lift of the hold here.

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