The treatment is an ex-vivo CRISPR genome-editing therapy that has had promising initial laboratory results, and the potential to be the first such therapies for sickle cell disease.
The next step is to carry out an early-phase trial of CTX001 in adult volunteers, however CRISPR Therapeutics has confirmed that the FDA has withheld approval of the study on the basis that there are a number of unanswered questions requiring further investigation.
'We're working with the FDA and will respond as soon as we can about whatever questions they have regarding the trial,' said Chris Erdman, a spokesman for the company.
Sickle cell disease is a serious genetic blood disorder in which red blood cells are distorted into a sickle shape due to the presence of abnormal haemoglobin. This impairs the transport of oxygen around the body and can lead to a multitude of health issues including chronic pain, blindness and strokes.
CTX001 uses genome editing to modify blood stem cells, which are taken from the patient, so that the cells produce increased levels of fetal haemoglobin. Typically only present in humans at birth, fetal haemoglobin binds oxygen at a greater affinity than adult haemoglobin and has been found to have a positive effect in sickle cell disease patients, reducing the symptoms of the disease and alleviating sickle cell crises. CTX001 is predicted to replicate the rare natural phenomenon of increased levels of fetal haemoglobin in adults.
The first human clinical trial of a therapy using CRISPR genome editing started in China in 2016 (see BioNews 878). However, the CTX001 trial would mark the first such trial to take place in the USA. There is also a Phase I/II trial planned in Europe to investigate the use of CTX001 in patients with beta-thalassaemia; this is currently unaffected with an expected start date later in the year (see BioNews 945).
Brian Skorney, a biotechnology analyst with Robert W. Baird & Co headquartered in Milwaukee, Wisconsin, told Bloomberg: 'There's so much promise with this technology, but it's such early days. It's a permanent alteration of a patient's genetic code so I think the FDA is going to be cautious.'