Accidental discovery also known as 'Serendipity', has played a pivotal role in the discovery of many drugs used today. Many important discoveries in medicinal chemistry are due to serendipity. Some Examples are given below:
Penicillin: It is the first discovered antibiotics. By 1927, Fleming was investigating the properties of staphylococci. On 3 September 1928, Fleming returned to his laboratory having spent August on holiday with his family. Before leaving, he had stacked all his cultures of staphylococci on a bench in a corner of his laboratory. On returning, Fleming noticed that one culture was contaminated with a fungus, and that the colonies of staphylococci that had immediately surrounded it had been destroyed, whereas other colonies farther away were normal. Fleming showed the contaminated culture to his former assistant Merlin Price, who reminded him, "That's how you discovered lysozyme. Fleming grew the mould in a pure culture and found that it produced a substance that killed a number of disease-causing bacteria. He identified the mould as being from the Penicillium genus, and, after some months of calling it "mould juice", named the substance it released penicillin on 7 March 1929.
Vaccine of Chicken Pox: Serendipitous connection of dots led to the discovery that vaccination is a general method for preventing infectious diseases. It occurred during Pasteur’s struggle to establish the germ theory of disease, specifically in the case of chicken cholera. To demonstrate that chicken cholera was caused by germs and not some toxins in the blood, it was important to isolate germs from toxins, which was very difficult in those early days. To purify germs, Pasteur extracted them from diseased chickens, grew them in cultures outside the body, and injected the culture into healthy chickens, which would become sick and a new source of germs, hopefully less contaminated with toxins. An autumn day in 1879, he injected a culture into some chickens and found them to be hardly affected. Later, because of a mix up or shortage of supply, the “used” chickens were recycled, together with some new chickens, in experiments to test a fresh virulent culture. This time the new chickens all died but the lucky recycled chickens remained unscathed. A colleague recalled that when Pasteur heard of this surprising development, he “remained silent for a minute, then exclaimed as if he had seen a vision: ‘Don’t you see that these animals have been vaccinated!
Sildenafil: Researchers at Pfizer had worked for years on sildenafil, an inhibitor of the PDE5 enzyme, which they hoped would be effective in relaxing coronary arteries and relieving chest pain. Their hope was dashed by 1992. Only one out of ten candidates entering clinical trials successfully passes through the gauntlet and reaches consumers, but statistics is little consolation for those who see their projects falling on the heap of nine. Gloomily, the researchers terminated the trial and asked participants to return the unused drug. Many men refused, clutching to the drug as if it was gold. Idiosyncrasies being present in all clinical tests, researchers gave the objection little thought until they heard rumors about the drug’s side effects on sex life and, more important, read a paper on the role of PDE5 in the chemical pathway of erection. Gloom evaporated in the excitement that sildenafil may be a blockbuster after all. This time, their expectation was confirmed by new clinical tests on impotent men. They stumbled on an effective drug for erectile dysfunction Pfizer would market as Viagra.