The drug development process (fig. 2) is a lengthy, high risk and costly endeavor; many strategies exist to accelerate the target to clinical candidate selection as well as to provide the highest quality of the candidate.
Fluorine and its isotope have many role in the different phases of drug development process. The number of fluorine containing drugs are growing rapidly which include the best selling drugs such as Atorvastatin, Prozac, Ciprobay and Pantoprazole (fig.1).
Target identification:
PET is a nuclear medicine imaging tool that allows three-dimensional quantitative determination of the distribution of radioactive with in the human body. The relatively long half-life, high % of β emission, and relatively low positron energy 18 F make it is most favorable for the Positron Emission Tomography (PET) studies. F MR - Fluorine Magnetic Resonance. allows detection of the presence of the target, in vivo, including assessment of the presence of targets, as well as quantification of their spatial and temporal distribution.
F MR - Fluorine Magnetic Resonance.
PET- Positron Emission Tomography.
Lead Finding: Once the target is chosen and identified, and the next stage is typically high-throughput screening of large libraries of chemicals for their ability to modulate the target. F M R allows compound screening using cell-based and animal-based assays (whereas HTS is restricted to cell-based assays). Fluorine plays an important role in the physicochemical properties (see lead optimization) of the molecule, so the HTS screening of fluorine-containing libraries will help for the lead finding.
Lead Optimization: The small and highly electronegative fluorine atom can play a role in medicinal chemistry. Systematic fluorine scan of ligands is a promising strategy in lead optimization. It not only helps to enhance the physicochemical properties but also to strengthen Protein-Ligand binding interaction. This would make the molecule a safer candidate.
The current strategies for introducing fluorine atoms into molecules are centered to
1. Improve metabolic stability,
2. Alter physicochemical properties such as pKa and lipophilicity, dipole moment, and even the chemical reactivity and stability of the neighboring functional groups,
3. Enhance the binding efficacy and selectivity in pharmaceuticals, and
4. Bioisosterism.
Preclinical and Clinical Studies: The suboptimal pharmacokinetics and pharmacodynamic can lead up to 40% of the drug candidate failing to make it to phase 1 trial. PET can allow assessment of parameters such as drug absorption biodistribution, metabolism, delivery, and dose uses in preclinical studies and can help in systematic planning latter phases. The estimation of pharmacological agents to reach their targets is important in drug trials. This can be done by the ADME techniques based on blood or tissue harvesting and subsequent drug and metabolite analysis. This approach is less than perfect because plasma levels of the compound often do not reflect concentrations in a specific tissue because of the presence of physicochemical barriers such as between blood and brain. Proton Emission tomography provides a reliable measure of tissue drug concentration.
References
1) Muller. K et al., Science. 317, 2007, 1881.
2) Reid G. D et al., Drug discovery today. 13, 2008, 473.
3) Willmann J. K. et al., Nature reviews drug discovery. 7, 2008, 591.
