Enantiomer: Definition and Relevance to Ketamine

What Is an Enantiomer?

An enantiomer is one of two mirror-image forms of the same molecule that share identical atoms and chemical bonds but cannot be superimposed on one another, much like a left hand and a right hand. This mirror-image property is called chirality, from the Greek word for hand, and the atom responsible for it is known as a chiral center or stereocenter. Ketamine has one chiral center, which produces two enantiomers: S-ketamine (esketamine) and R-ketamine (arketamine).

Because receptors, enzymes, and transporters in the body are themselves chiral, they can respond very differently to each enantiomer of a drug. A receptor binding pocket fits one spatial arrangement more precisely than its mirror image, similar to how a glove fits one hand better than the other. As a result, S-ketamine and R-ketamine differ in NMDA receptor binding strength, side effect profile, metabolism, and in some cases therapeutic effect, even though the two molecules share the same chemical formula.

Chirality in Chemistry

The Chiral Center

Most chiral drug molecules contain a carbon atom bonded to four different chemical groups. This carbon is the chiral center, and it creates the possibility of two distinct spatial arrangements of the same molecule. Chemists label these arrangements using the R/S naming system, based on the priority of the attached groups, or describe them by how they rotate polarized light: dextrorotatory (+) for clockwise rotation or levorotatory (-) for counterclockwise rotation.

Racemic Mixtures

When a chiral drug is synthesized using standard chemical methods, the reaction typically produces a racemic mixture, an equal 50:50 blend of both enantiomers, marked with the prefix (±). Separating a racemic mixture into individual enantiomers requires specialized techniques such as chiral chromatography or enzymatic resolution. Racemic ketamine, the form used in most IV infusion clinics and compounded oral or sublingual formulations, is exactly this kind of 50:50 blend of S-ketamine and R-ketamine.

S-Ketamine vs. R-Ketamine

S-ketamine, also called esketamine, binds the NMDA receptor's PCP binding site more tightly than R-ketamine. According to a 2018 pharmacology review in Pharmacological Reviews, S-ketamine shows several times greater affinity for the NMDA receptor than R-ketamine, one reason S-ketamine is generally considered the more potent enantiomer at equivalent doses (Zanos et al., 2018).

The U.S. Food and Drug Administration approved esketamine nasal spray, brand name Spravato, in March 2019 for treatment-resistant depression when used alongside an oral antidepressant (FDA). Spravato contains only the S-ketamine enantiomer, isolated from the racemic mixture used in IV or compounded ketamine. See our esketamine glossary entry and the racemic ketamine vs. esketamine comparison for more on dosing, delivery, and clinical use.

R-ketamine, or arketamine, binds the NMDA receptor less tightly and has historically been considered less potent as a dissociative anesthetic. Researchers have studied R-ketamine for its potential to produce antidepressant effects with fewer dissociative side effects than S-ketamine, though R-ketamine has not received FDA approval and remains investigational. Because the two enantiomers differ in receptor binding, metabolism, and side effect profileesketamine and racemic ketamine are not interchangeable at the same milligram dose, and providers adjust dosing protocols accordingly.

Why the Enantiomer Distinction Matters for Patients

The enantiomer question is not just academic. Ketamine treatment for depression, anxiety, and chronic pain is delivered in several different forms, and each form uses a different balance of S-ketamine and R-ketamine. IV ketamine and most compounded oral or sublingual products use racemic ketamine, the 50:50 mixture of both enantiomers. Spravato uses only the S-ketamine enantiomer at an FDA-approved dose and delivery method. See how ketamine works in the brain and ketamine therapy safety considerations for more on how formulation affects the treatment experience.

Because potency differs between enantiomers on a milligram-for-milligram basis, a dose appropriate for racemic ketamine is not automatically appropriate for esketamine, and vice versa. Patients comparing a racemic ketamine provider with an esketamine provider, or switching between the two, should confirm which enantiomer or mixture a specific treatment uses rather than assuming the dosing or side effect profile will match.