The impact of computer-aided drug designing in the field of medicinal chemistry has created a boon in the drug discovery process. Molecular docking is an integral part of bioinformatics that deals with protein-ligand interactions, binding conformations, and affinity predictions. It has shown to be a rapid, easy, and affordable method in business and research settings alike. The advancement in the hardware and software has led to enhanced molecular dynamic simulations and thus authenticate the computational results. This has created a great impact in minimizing the cost and time involved in the drug discovery process. It has also helped in identifying the rationale for drug repurposing approaches. This chapter will give in-depth knowledge of the importance of molecular docking in drug designing and discovery, their impact on drug repurposing, and success stories of the in silico approach in drug discovery and repurposing.

Background: Primary amoebic meningoencephalitis (PAM) is an acute and fulminant CNS infection caused by Naegleria fowleri. Recreational activities and ritual ablution with contaminated warm fresh water are the main reason of PAM. Pakistan ranked the second most affected country, where most of the PAM incidences were reported from Karachi, Pakistan. Methods: In May, 2019, a 28-yr-old suspected PAM patient came to the Imam Zain-Ul-Abdin Hospital, Karachi. Biochemical and cytological investigations of patient`s CSF were carried out at Karachi Diagnostic Center and Molecular Biology Lab. Sequencing of Naegleria sp. specific (ITS) primer-based amplicons was performed from both patient`s CSF and water samples followed by multiple sequence alignment and phylogenetic studies. Results: Biochemical and cytological investigations of patient`s CSF showed 5 mg/dl glucose, 240 mg/dl total protein and 2260/mm3 TLC suggesting acute meningoencephalitis. PCR-based analyses of patient`s CSF and his residential tap water samples using Naegleria sp. specific (ITS) and N. fowleri specific primers revealed the presence of N. fowleri DNA. Nucleotide sequences of ITS primer-based amplicons from both patient`s CSF and water samples were submitted in GenBank under the accession numbers MT726981.1 and MT726226.1, respectively. According to phylogenetic analysis, N. fowleri isolate from Pakistan has shown the least node age of seven. Conclusion: Here, for the very first time in Pakistan, N. fowleri genotype has been identified as type-2. Phylogenetic analysis showed that N. fowleri isolate from Pakistan is among the latest descendants, i.e., evolved later in life.

Naegleria fowleri, also known as brain-earing amoeba, causes severe and rapidly fatal CNS infection in humans called primary amebic meningoencephalitis (PAM). The DNA from the N. fowleri clinical isolate was sequenced for circular extrachromosomal ribosomal DNA (CERE - rDNA). The CERE contains 18 S, 5.8 S, and 28 S ribosomal subunits separated by internal transcribed spacers, 5 open reading frames (ORFs), and mostly repeat elements comprising 7268 bp out of 15,786 bp (46%). A wide variety of variations and recombination events were observed. Finally, the ORFs that comprised only 4 hypothetical proteins were modeled and screened against Zinc drug-like compounds. Two compounds [ZINC77564275 (ethyl 2-(((4-isopropyl-4 H-1,2,4-triazol-3-yl) methyl) (methyl)amino) oxazole-4-carboxylate) and ZINC15022129 (5-(2-methoxyphenoxy)-[2,2’-bipyrimidine]-4,6(1 H,5 H)-dione)] were finalized as potential druggable compounds based on ADME toxicity analysis. We propose that the compounds showing the least toxicity would be potential drug candidates after laboratory experimental validation is performed.