"Nature is not neutral." Explain how homochirality in biomolecules illustrates this, and what it means for drug development and regulation.

In 1848, Louis Pasteur showed that molecules can exist as mirror-image forms called enantiomers, which cannot be laid exactly on top of each other. Life does not treat the two forms equally. Biology is homochiral, which means it uses only one form. This preference is total (100%), not a slight imbalance [1]. That one-sided preference shapes how medicines must be designed and regulated.

How homochirality shows nature is not neutral

• One-handed building blocks: every chiral amino acid used by life is left-handed (L), and every chiral sugar, including those in DNA and RNA, is right-handed (D) [1].
• Lab vs life: ordinary chemical synthesis usually gives a 50:50 racemic mixture of both forms. Living systems keep only one, which points to selection, not chance.
• A bias older than life: NASA Goddard found extra L-isovaline in carbon-rich meteorites. The excess was larger in meteorites that had been altered by water [2]. A small tilt from space may later have been amplified into total one-handedness.
• A sign of life: because the preference is absolute, NASA scientists study homochirality as a possible biosignature for detecting life beyond Earth [1].

Implications for drug development

• Mirror forms act differently: the body's receptors and enzymes are themselves chiral, so they tell enantiomers apart. One form may treat a disease while its mirror form is inactive or toxic [3].
• Thalidomide lesson: the drug was sold as a mixture of both forms, and its birth defects are linked to the (S)-form. A pure sample also turns back into a mixture in blood serum, with a half-life of about 1 hour [4]. A drug that is pure in the factory may not stay pure in the patient.
• Design response: develop single-enantiomer drugs and test whether they stay stable inside the body, not just whether they are pure in the vial.

Implications for regulation

• Each mirror form is a separate drug: the US FDA's 1992 policy asks for early stereospecific assays (tests that measure each form separately) and an assessment of whether the forms convert into each other in the body [3].
• India: new drug approvals under the NDCT Rules, 2019 [5] could set out explicit chirality-specific requirements, such as toxicity data for each form and evidence of stability in the body. This would align CDSCO with global practice.

Homochirality shows that nature's preferences are built into life itself. Medicine has to work with this bias, not assume both mirror forms behave alike. Treating each form as a separate compound, testing whether it stays stable in the body and bringing Indian rules in line with the best global standards would make chiral drugs safer. This would advance SDG 3 (Good Health and Well-being).