Moving a drug from the laboratory to the clinic is terrifically expensive and slow. Even after extensive tests in cultured cells and laboratory animals, the drug may still fail in its clinical tests. Such failures cost drug companies massive amounts of money, and this drives up the cost of those drugs that succeed in clinical trials and secure FDA approval. If scientists could design drug assays that better predict whether a compound will succeed in human trials could help pharmaceutical companies identify the most promising drugs in which to invest their resources.
Fortunately, a study published this week in Cell Stem Cell might represent such a breakthrough. In this paper, researchers reported that a new stem cell-based assay was actually able to pinpoint a potential small molecule treatment for amyotrophic lateral sclerosis (ALS, also known as known as Lou Gehrig’s disease). In follow-up experiments, the drug promoted better cell survival better than two other drug candidates that had recently failed in phase III clinical trials.
One of the study’s co-authors, Clifford Woolf, the director of the F. M. Kirby Neurobiology Center at Boston Children’s Hospital, said that this new strategy “could either be used in the late preclinical stage to confirm the cellular actions of particular leads, or even better as a driver of early exploratory preclinical testing, revealing new targets and pathways.”
The laboratory of Lee Rubin at Harvard Medical School developed this new cell-bases assay by using embryonic stem cells derived from both healthy mice and those with a mutation in the gene SOD1. Mutations in SOD1 are known to cause ALS in people. After differentiating the stem cells into motor neurons, which are the cells that die off in ALS patients — the group exposed these embryonic stem cell-derived motor neurons to 5000 different small molecular-weight compounds. The cultured cells were also deprived of essential chemicals from the culture medium in order to accelerate their death.
In these experiments, a molecule called kenpaullone, which is an inhibitor of the enzyme GSK-3, stood out in their initial screen. GSK-3 controls cell growth and death, and kenpaullone strongly promoted the survival of both normal and mutated motor neurons and kept them morphologically healthy. In a different experiment, the group showed that the drug decreased levels of SOD1, which is thought to aggregate in the motor neurons of people with the disorder.
In further tests, Rubin and his colleagues treated motor neurons made from induced pluripotent stem cells derived from adult cells that had been taken from two ALS patients with kenpaullone. One of these ALS patients had mutations in the SOD1 gene, while the other harbored mutations in the TDP-43 gene. TDP-43 is yet another gene associated with ALS, since mutations in it also cause ALS. Rubin’s team found that the small molecule substantially boosted motor neuron survival by some 2- to 4-fold, and this effect was dose-dependent and was observed in both healthy and diseased cells.
In contrast, two drug candidates that recently failed phase III clinical trials were less effective when tested in the same assay. The drug dexpramipexole had no effect on patient-derived motor neurons, and olesoxime had a variable but only moderately positive effect.
It’s not clear how the SOD1 mutation causes the degeneration of motor neurons. According to Alysson Muotri, assistant professor of pediatrics and cellular and molecular medicine at the University of California, San Diego, who was not involved in the new study; knowing the pathological mechanism of inactive SOD1 on motor neurons could inform additional endpoints for the stem cell assays
To date, kenpaullone has only been tested in on cultured neurons and not in living mice to date. And, as with all cell culture assays, it is an open question as to “the extent to which changes in neurons in a dish phenocopy complex diseases that may take many years to manifest, and if rescue of the phenotype by a hit in a screen will translate into therapeutic benefit in patients,” Woolf noted.
However Muotri sees great potential for stem cell-based assays and their use for drug discovery. “Stem cell based screens will definitely speed up drug discovery, bringing more powerful candidates to clinical trial,” said Muotri. “I can see this going into personalized medicine—we will be testing drugs and doses in motor neurons derived from each patient to personalize treatment.”
Yang, Y. M., S. K. Gupta, K. J. Kim, B. E. Powers, A. Cerqueira, B. J. Wainger, H. D. Ngo, K. A. Rosowski, P. A. Schein, C. A. Ackeifi, et al. 2013. A small molecule screen in Stem-Cell-derived motor neurons identifies a kinase inhibitor as a candidate therapeutic for ALS. Cell Stem Cell (April).