Drug Repurposing Screens: Redeploying Old Dogs for New Tricks

This blog was written by guest author, Amy Landreman, PhD.

Drug repurposing, identifying new uses for approved or investigational drugs, is an attractive strategy when looking for new disease treatments. Because the compounds have already gone through some level of pre-clinical optimization and safety testing, this approach can reduce risk, reduce cost, and speed up the timeline for further drug development. An additional benefit of this approach is that it can result in new biological insights or a better understanding of disease mechanisms since these compounds usually already have some level of mechanistic characterization. Indeed, there are now a number of compound collections openly available specifically for the purpose of facilitating drug repurposing efforts. For example, the ReFRAME (Repurposing, Focused Rescue, and Accelerated Medchem) library is a collection of 12,000 compounds developed by Scripps Research Center and has been screened to identify novel candidate therapeutics for Cryptosporidium infection (1). The Broad Institute also offers a drug repurposing hub that contains an annotated collection of over 7,000 compounds.

Drug repurposing libraries, although often smaller than novel compound small molecule libraries, are designed for implementation into high-throughput screening workflows in order to efficiently triage compounds for the desired result. Effective compound screens require assays that can be scaled to 384 or 1536-well microplate formats and implemented in batch or continuous processing workflows. The firefly luciferase reaction has been leveraged to create many assays that are well-suited to these types of high-throughput screening approaches. In particular, the generation of “Glow” assays that have stable luminescent signals and homogenous assay design is a good fit. The signal stability allows for multi-plate processing and because the reagent is added directly to cells in culture, pre-processing steps are eliminated allowing for automated workflows. Assay reagents such as the CellTiter-Glo® Cell Viability Assay and the ADP-Glo™ Kinase Assay are commonly used in screening efforts including those done with repurposing libraries.  In addition, there are several firefly luciferase reporter assay reagents such as Steady-Glo® and Bright-Glo™ Luciferase Assays that have been optimized for high-throughput detection of firefly luciferase activity making them well-suited to repurposing screens.

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From Viral Outbreak to Vaccine Development: Our Top 10 Most Viewed Blog Posts of 2020

This illustration, created at the Centers for Disease Control and Prevention (CDC), reveals ultrastructural morphology exhibited by coronaviruses. Photo Credit:  Alissa Eckert, MS; Dan Higgins, MAM CDC It is one is used in several of our top 10 most viewed blogs of 2020
Illustration from CDC; Photo Credit: Alissa Eckert, MS; Dan Higgins, MAM

When you look at our top 10 most viewed blog posts of 2020, there’s no surprise that all relate to COVID-19. We have come a long way since the beginning of the year, thanks to tireless scientists and researchers around the globe. They have led the way in COVID-19 research, treatment, and testing. Let’s take a closer look at this top 10 list:

10. Tips to Maintain Physical Distance in the Lab 

The spread of COVID-19 forced us to adapt and adjust to new ways in life, in work, and for this blog post, in the lab. In response to the pandemic, some labs shut down completely. Others have stayed open, especially those involving coronavirus research. This post provides 10 helpful distancing tips for researchers to stay safe and productive while working in the lab.  

9. Investigation of Remdesivir as a Possible Treatment for SARS-2-CoV (2019 nCoV) 

Scientists have worked hard to determine possible treatment for COVID-19. This blog post focuses on Remdesivir (RDV or GS-5734), an encouraging treatment used for the first case in the United States. It provides an in-depth look at numerous studies and clinical trials on Remdesivir as treatment for COVID-19. One key finding is that RDV needed to be administered either before or shortly after infection to limit lung damage. 

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Choices for Measuring Luciferase-Tagged Reporter Pseudotyped Viral Particles in Coronavirus Research

Coronavirus (CoV) researchers are working quickly to understand the entry of SARS-CoV-2 into cells. The Spike or S proteins on the surface of a CoV is trimer. The monomer is composed of an S1 and S2 domain. The division of S1 and S2 happens in the virus producing cell through a furin cleavage site between the two domains. The trimer binds to cell surface proteins. In the case of the SARS-CoV, the receptor is angiotensin converting enzyme 2. (ACE2). The MERS-CoV utilizes the cell-surface dipeptidyl peptidase IV protein. SARS-CoV-2 uses ACE2 as well. Internalized S protein goes though a second cleavage by a host cell protease, near the S1/S2 cleavage site called S2′, which leads to a drastic change in conformation thought to facilitate membrane fusion and entry of the virus into the cell (1).  

CDC / Alissa Eckert, MS; Dan Higgins, MAMS

Rather than work directly with the virus, researchers have chosen to make pseudotyped viral particles. Pseudotyped viral particles contain the envelope proteins of a well-known parent virus (e.g., vesicular stomatitis virus) with the native host cell binding protein (e.g., glycoprotein G) exchanged for the host cell binding protein (S protein) of the virus under investigation. The pseudotyped viral particle typically carries a reporter plasmid, most commonly firefly luciferase (FLuc), with the necessary genetic elements to be packaged in the particle. 

To create the pseudotyped viral particle, plasmids or RNA alone are transfected into cells and the pseudotyped viruses work their way through the endoplasmic reticulum and golgi to bud from the cells into the culture medium. The pseudoviruses are used to study the process of viral entry via the exchanged protein from the virus of interest. Entry is monitored through assay of the reporter. The reporter could be a luciferase or a fluorescent protein.   

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