Biodistribution
Biodistribution generally refers to the tracking of molecules after they have been introduced into cells or tissues. In transfection experiments, biodistribution relates to tracking the locations of transfected nucleic acids and establishing where exactly they are transcribed or take effect. For example, in stable transfections, it is important to find any off-target effects that are caused by matching sequences in genomic DNA and transfected nucleic acids. In other intracellular contexts, biodistribution can involve tracking the organelles that a compound ends up in after being introduced into a cell. A popular example of biodistribution is in PET (positron emission tomography) scans, where a radioactive isotope is bound to a peptide in order to visualize physiological activity in the body.
In larger scale testing environments, such as mice or cultured tissues, biodistribution refers to the tracking of compounds as they migrate within tissues. Screenings can help show the paths that compounds take, which can provide clues to compound efficiency and general characteristics of tissue functionality. Thus, biodistribution is an important topic to drug research, as it always provides information on how efficiency is derived from intra- and inter-cellular processes.
siRNA Biodistribution and Screening
Introduction of synthetic siRNA via transfection causes a transient alteration in gene expression for a specific gene of interest. By synthesizing a siRNA mimic to each gene, RNAi may be utilized in a systematic approach to silence every gene in a cell, one gene at a time. For this reason, large scale screens can aid in mechanism of action determination, pathway analysis, anti-proliferation studies, etc. by exploitation of the RNAi pathway. Highly efficient animal siRNA delivery kits are typically nanoparticle– and liposome-based siRNA in vivo transfection reagents.
In a typical animal study, post-treatment tissue samples are collected from the animal for RNA isolation and gene quantification (e.g. qRT-PCR or western blot). Resection of multiple tissues enables researchers to establish the efficacy and global biodistribution of their therapeutic compound. qRT-PCR assays designed to detect the delivered siRNA or miRNA show delivery of the active pharmaceutical ingredient (API) to tissues throughout the body of the animal. However, keep in mind that delivery to a tissue does not necessarily mean the delivered cargo is active; changes in target gene expression should be monitored as well.
Biomarkers
Biomarkers are important to biodistribution as they are the indicators of compound location. For example, attaching a dye or fluorescent protein, such as GFP, to a compound can allow for quantitative analysis of where the compound goes once in a target biological region. Such quantitative analyses are important to both drug screenings and research into the pathways that drive compound absorption in tissue samples and cells.
Biomarker discovery refers to the use of protein, mRNA, or miRNA samples to determine differential expression of disease versus normal cells, or perhaps a treated versus non-treated sample. With the advent of high throughput sequencing and other quantitative tools, pharmaceutical companies are using these new technologies to find new druggable targets, as well as determining ways to increase efficacy of current and orphan drugs. The discovery of biomarkers is another service laboratories are offering assistance with. For certain experiments, biomarkers may not be in the class of those commonly used in research, and may require advanced techniques for detection. In these cases, specialized laboratory equipment should be used, or laboratory assistance should be obtained.
New research has used circulating RNAs (circRNA) as biomarkers to help diagnose diseases like lung adenocarcinoma, which is a sub-type of non-small cell lung cancer. Current diagnostic measures for lung cancer require imaging lung tissue, which is detected mostly when there is already a large, proliferating tumor mass. This builds the need for diagnostic techniques that can detect lung cancer at its early stages to prevent tumor progression. Studies have compiled data from research to determine two upregulated and two downregulated circRNA’s related to lung adenocarcinoma that have significantly differing levels than normal, healthy cells, showing correlation to worsened outcomes (specifically with hsa_circ_0005962). Despite the lack of knowledge regarding the respective mechanisms of action for cancer and circRNAs, the research offers increased evidence that circRNA’s are related to tumor prognosis (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6685864/)