Comparative Bioavailability Study of Ketamine and Norketamine After Single Dose Administration of Ketamine 10, 20, 40 and 80 mg Prolonged Release Tablets in Fasting State and 5 mg Ketamine Solution for Infusion in 15 Healthy Subjects in Serum, Urine and Feces

Ketamine, a cyclohexanone derivative, is a non-barbiturate anesthetic agent which has been available in clinical practice for more than 40 years. Ketamine was first synthesized in 1962 and patented in Belgium in 1963. Ketamine, a potent non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, has a recognized unique therapeutic value in human and veterinary medicine as an anesthetic with analgesic properties. Ketamine is considered as one of the World Health Organisation (WHO) essential drugs for the management of refractory pain.

Ketamine is widely used for the induction and maintenance of general anesthesia, usually in combination with a sedative and if required a muscle relaxant. When used as an intravenous (i.v.) or intra-muscular (i.m.) injection, ketamine is best suited for short procedures. With additional doses, or by i.v. infusion, ketamine can be used for longer procedures. Due to its pharmacodynamic properties, important clinical applications are mainly brief diagnostic ones and surgical procedures in paediatric patients and ambulatory anesthesia, treatment of burn patients, obstetrics, and for the induction and maintenance of anesthesia in trauma victims, hypovolemic patients, patients with septic or cardiogenic shock, and patients with pulmonary diseases. Other uses include sedation in the intensive care setting, and analgesia particularly in emergency medicine as well as treatment of bronchospasm / asthma. However, the use of ketamine for humans in the EU is restricted to special indications, due to the occurrence of emergence reactions. Outside the EU, its ease of use gives ketamine a major advantage under difficult circumstances (developing countries and remote areas).

For therapeutic purposes, ketamine is usually administered intravenously (i.v.) or intramuscularly (i.m.). Its hydrochloride salt is marketed under many brand names such as Ketanest, Ketalar, or Ketaset. Ketamine is a chiral compound. While most marketed ketamine preparations are racemic mixtures of R-(-)-ketamine and S-(+)-ketamine (1:1), formulations containing only the enantiomer S-(+)-ketamine are also commercially available (e.g. Ketanest® S, Pfizer Pharma GmbH). S-(+)-ketamine is approximately four times more potent than R-(-)-ketamine and twice as potent as the racemic mixture.

It is well-known that ketamine exerts analgesic effects at subanesthetic doses. Therefore, during the past 15-20 years, ketamine has been used as an analgesic in a wide range of pain settings, including acute (e.g. post-operative pain, pain during dental procedures) and chronic pain (e.g. central or peripheral neuropathic pain, cancer pain) conditions.

Although the drug is only approved for intramuscular injection and intravenous injection or infusion, ketamine is frequently used via other routes of administration, such as subcutaneous (s.c.) injection or infusion, rectal, oral, intranasal, transdermal (patch), intrathecal or epidural administration. Considering that no oral formulation is approved, the marketed solution for injection/infusion or an extemporaneous preparation was used in clinical studies investigating the efficacy of orally administered ketamine. For clinical (off-label) use, the solution for injection/infusion is generally employed for oral treatment.