Heterocyclic compounds and their roles in biochemical sciences and therapeutic uses

Keisham Surjit Singh *

A heterocyclic compound is a cyclic compound that contains a hetero atom ie it must have at least one element other than carbon as a member of its ring.

The prefix hetero - is derived from the Greek word-heteros, meaning “other” or “different” and it refers to the non-carbon atoms or hetero-atoms in the ring. The presence of the hetero atoms gives heterocyclic compounds unique physical and chemical properties that are often quite distinct from those of their all-carbon-ring analogues.

Some examples of naturally occurring heterocyclic compounds include pyridine, pyrimidine, quinoline, isoquinoline, imidazole, quinine, essential amino acids, vitamins, thiamine, riboflavin etc. In the last few decades, quite a large number of heterocyclic compounds containing three, four, five and six-member rings were synthesized in laboratory by many chemists for their use in various purposes.

The most common hetero atoms in heterocyclic compounds are nitrogen (N), sulphur (S) and oxygen (O). Heterocyclic compounds are found in plants and animal products; and they are one of the important constituents of almost one-half of the natural organic compounds known eg, alkaloids, natural dyes, drugs, proteins, enzymes, etc.

These compounds are also important components of many of the biochemical materials that are essential to life. For instance, nucleic acids (the chemical substances that carry the genetic information controlling inheritance) consist of long chains of heterocyclic units held together by chemical bonds.

The commonly used popular anti-malaria drug quinine is also an alkaloid (heterocyclic compound) derived from the bark of the cinchona tree that have been used for treatment of malaria since before 1633. Quinine is used to treat lifethreatening infections caused by chloroquine-resistant, Plasmodium falciparum malaria which are spread to human beings through the bites of infected female Anopheles mosquitoes.

The anti-malaria drugs such as chloroquine and hydroxy chloroquine are also heterocyclic compounds that have been used for treatment of malaria. Malaria is a global infectious disease that remains a leading cause of morbidity and mortality in the developing world like South Asian and African countries.

Severe and fatal malaria is predominantly caused by Plasmodium falciparum. Quinine is used as a sch-izonticidal drug (a drug that can disrupt different parasite stages and as a result of the disruption or blockage of the life cycle stage, the parasite is unable to complete its life cycle to increase its number within-host body) and is found to be less effective and more toxic than chloroquine.

The mechanisms of anti-malarial activities of quinine are not well understood. However, it has a special place in the management of severe falciparum malaria in areas with known resistance to chloroquine.

During COVID-19, some countries recommended hydroxychloroquine for the treatment of COVID-19 at the initial stage although there were many conflicting reports for the efficiency and side effects of these drugs for COVID-19 treatment. However, the World Health Organization (WHO) did not recommend this drug for the treatment of the disease during the crisis.

In addition, many naturally occurring compounds like pigments, vitamins, and antibiotics are also heterocyclic compounds. Over the years, heterocyclic compounds become an indispensable part of our day-to-day life and nowadays, modem society is dependent on synthetic heterocycle compounds for their use as drugs, pesticides, dyes, plastics etc.

Heterocyclic compounds are also important for medicinal chemists and they possess various therapeutic properties including antifungal, anti-inflammatory, anti-tumor, anti-malarial, antioxidant and analgesic activities.

In summary, compounds derived from heterocycles find applications in pharmacy, medicine, agriculture, plastics, polymers, and other areas. Due to their potential biological activities, heterocyclic compounds are being employed in the treatment of various infectious diseases. Various heterocycle compounds that have been prepared in laboratories are also successfully used as clinical agents.