Final answer:
Eukaryotic cells like fungi and protozoa share various cellular structures with human cells, making selective drug toxicity challenging. Viruses also complicate drug design as they replicate within human cells. Despite these challenges, research has led to the development of drugs with selective toxicity, utilizing cell lines and in vitro/in vivo correlations.
Step-by-step explanation:
Selective drug toxicity is difficult to achieve with eukaryotic pathogens such as fungi, protozoans, and helminths because they share cellular characteristics with human cells. Drugs that target prokaryotic cells, like bacteria, have clear targets because bacterial cells differ significantly from human cells. However, with eukaryotic cells having similar ribosomes, cytoskeletons, and cell membranes, finding a target that doesn't harm human cells is much more complex. This is exemplified in the difficulty of creating antifungal drugs without affecting human cells.
Working with cell lines, including immortalized cell lines, is part of the research process for developing these drugs. Cell lines provide a stable, consistent, and potentially infinite supply of cells for study. This is crucial for testing the selective toxicity of new compounds both in vitro and against in vivo models. Validation against in vivo models ensures that in vitro findings are predictive and relevant to real-world applications.