Oxytocin Pharmacokinetics After Intramuscular Injection

In this study people with knee arthritis so severe that a joint replacement is needed are recruited for a one day study. Study participants will have an intravenous catheter (IV) inserted in the forearm. A single intramuscular (IM) injection of oxytocin (Pitocin®), 17 micrograms will be administered and blood samples will be taken several times over the next 120 minutes. The amount of oxytocin will be measured in the blood samples.

The main purpose of this study is to sample the blood, before and after the administration of oxytocin and calculate the pharmacokinetics (amount of oxytocin in the blood over time) of oxytocin.

This is an unblinded, sequential study of subjects, all will receive an intramuscular (IM) injection oxytocin with blood samples taken thereafter in order to create a formula to describe the concentrations of oxytocin in the blood over time (pharmacokinetics). In this study people with knee arthritis so severe that a joint replacement is needed are recruited for a one day study. Participants will come to the Clinical Research Unit (CRU) and have an intravenous catheter (IV) inserted in the forearm. Participants will get a single IM injection of oxytocin (Pitocin®), 17 micrograms (10 International Units (IU)) and blood will be taken several times over the next 120 minutes and the amount of oxytocin measured in the blood samples. This information will be analyzed by another group at Stanford University in the pharmacokinetics/pharmacodynamics (PK/PD) Core part of this application. The Core group at Stanford will use mathematics to fit the amount of oxytocin over time as it disappears from blood to a formula, called pharmacokinetics. The effect of subject age, sex, race, ethnicity and weight on the pharmacokinetics of oxytocin will be tested for, since these things can affect how quickly drugs circulate in the blood and are important to better adjust the dose of drug to the individual.

The main purpose of this study is to sample the blood and calculate the pharmacokinetics of oxytocin. The research participants will not benefit from this study, but the knowledge investigators get will be important not only to adjust oxytocin dose to individuals, but to study its possible effects on pain in a very standardized way. The sample size chosen is needed to get an accurate estimate for the parameters in the pharmacokinetic model for the population, not just the subjects in this study.

Eligibility Criteria:
Inclusion Criteria:

* 1. Male or female \> 18 and \< 75 years of age, Body Mass Index (BMI) \<40.
* 2. Generally in good health as determined by the Principal Investigator based on prior medical history, American Society of Anesthesiologists physical status 1, 2, or 3.
* 3. Normal blood pressure or, for those with hypertension, pressure controlled with anti-hypertensives and with a resting heart rate 45-100 beats per minute.
* 4. Female subjects of child-bearing potential and those \< 1 year post-menopausal, must be practicing highly effective methods of birth control such as hormonal methods (e.g., combined oral, implantable, injectable, or transdermal contraceptives), double barrier methods (e.g., condoms, sponge, diaphragm, or vaginal ring plus spermicidal jellies or cream), or total abstinence from heterosexual intercourse for a minimum of 1 full cycle before study drug administration.
* 5. diagnosis of advanced knee arthritis

Exclusion Criteria:

* 1. Hypersensitivity, allergy, or significant reaction to any ingredient of Pitocin®
* 2. Any disease, diagnosis, or condition (medical or surgical) that, in the opinion of the Principal Investigator, would place the subject at increased risk (active gynecologic disease in which increased tone would be detrimental e.g., uterine fibroids with ongoing bleeding), compromise the subject's compliance with study procedures, or compromise the quality of the data
* 3. Women who are pregnant (positive result for serum pregnancy test at screening visit), women who are currently nursing or lactating, women that have been pregnant within 2 years
* 4. Subjects with neuropathy, chronic pain, diabetes mellitus, or taking benzodiazepines or pain medications on a daily basis.