What happened

Shares of CRISPR Therapeutics (NASDAQ:CRSP) rose over 113% last year, according to data provided by S&P Global Market Intelligence. The pharma stock built momentum throughout much of the year, but surged in October ahead of an important data presentation that ultimately lived up to the hype. That allowed the gene-editing stock to easily outperform the 28.8% gain of the S&P 500 in 2019.

The end-of-year rally was driven by promising clinical results for its lead drug candidate. The first two individuals, one with sickle cell disease (SCD) and one with transfusion-dependent beta thalassemia (TDT), dosed with CTX001 achieved functional cures after receiving an initial dose of the gene-editing product. The results need to be proven durable and replicated in a larger number of patients, but the update was about as good as investors could have hoped for at the current stage of development.

A businessman tossing cash into the air.

Image source: Getty Images.

So what

Both SCD and TDT are caused by structural abnormalities in red blood cells. But these are one of the few cells in the human body that don't contain DNA. That means CRISPR Therapeutics has to harvest stem cells from the bone marrow of patients, apply gene editing to those extracted cells, and then inject the engineered stem cells back into patients (the ex vivo method). If the therapy works, then the engineered stem cells should produce functional red blood cells and potentially result in a cure.

In the early study, the ex vivo approach of CTX001 appeared to do just that. The TDT patient required an average of 16.5 blood transfusions per year in the two years before the clinical trial. Nine months after receiving the gene-editing treatment, the individual was transfusion independent (compared with an expected 12 transfusions) and expressed working copies of hemoglobin on 99.8% of red blood cells. 

The SCD patient experienced an average of seven vaso-occlusive crises (painful blockages of blood vessels caused by abnormally shaped red blood cells) per year in the two years before the clinical trial. Four months after receiving the gene-editing treatment, the individual reported no vaso-occlusive crises (compared with an expectation for two such episodes) and expressed working copies of hemoglobin on 94.7% of red blood cells.

Now what

The early success of CTX001 bodes well for the ex vivo approach of CRISPR Therapeutics and its partner Vertex Pharmaceuticals (NASDAQ:VRTX), but investors should be careful not to extrapolate the results too broadly. Gene-editing tools that are applied inside the body (in vivo) face significantly steeper obstacles, such as the difficulty of delivering gene-editing payloads to specific tissue types inside the body. There's also the elephant in the room: Scientists are beginning to realize that current-generation CRISPR gene-editing tools don't work all that well

Nonetheless, CRISPR Therapeutics is the top CRISPR-based gene-editing stock on the market. It has the cash, the partnerships, and the early results to back up its claim to that label.