Study of Pyridostigmine With Ondansetron in Subjects With Anti-AchR Positive Myasthenia Gravis

Inclusion criteria

1. Male and female subjects aged 18 years old and up who have been diagnosed withanti-AchR antibody positive myasthenia gravis.

2. Subjects can be either: currently taking pyridostigmine and after Lead-in Period experience dose limiting GIAEs related to pyridostigmine (must include nausea, vomiting or diarrhea) duringLead-in

3. Subjects must be willing and able to complete a GI symptom diary within a consistenttimeframe on a daily basis.

4. Must be clinically stable in judgement of treating neurologists for past 3 months.

5. Must be positive for acetylcholine receptor antibodies.

6. Subjects must be able to swallow liquid.

7. Subjects must be in otherwise good health as determined by their medical history,physical examination, vital signs, and laboratory tests. A subject with a medicalabnormality may be included only if the investigator or designee considers that theabnormality will not introduce significant additional risk to the subject's healthor interfere with study objectives.

8. Subjects must have signed an informed consent form indicating that they understandthe purpose of, and procedures required for the study and are willing to participatein the study and comply with the study procedures and restrictions.

Exclusion criteria:

1. Any acute or chronic diseases which are associated with GI distress (such as nausea,vomiting, or diarrhea), which could interfere with the subjects' safety during thetrial, expose them to undue risk, or interfere with the study objectives.

2. History or presence of hepatic, or renal disease or other condition known tointerfere with the absorption, distribution, metabolism, or excretion of drugs.

3. History of substance abuse, known drug addiction, or positive test for drugs ofabuse or alcohol.

4. Patients currently using marijuana for any reason (medical or recreational).

5. Known hypersensitivity to pyridostigmine, or to ondansetron or similar 5-HT3serotonin receptor antagonists.

6. ECG changes including QT interval prolongation and congenital long QT syndrome.Electrolyte abnormalities (e.g., hypokalemia or hypomagnesemia), congestive heartfailure, bradyarrhythmia's or other medicinal products that lead to QT prolongation.

7. Treatment with drugs affecting peripheral cholinergic transmission within 1 month ofstudy entry (with the exception of pyridostigmine).

8. Subjects unlikely to co-operate during the study, and/or be questionably compliantin the opinion of the investigator.

9. Patients currently being treated with narcotics.

10. Patients being treated with aminoglycoside antibiotics, which are contraindicated inmyasthenia gravis.

11. Patients unable to be contacted in case of an emergency.

12. Intake of an investigational drug within 30 days of study entry.

13. Pregnancy and women of childbearing potential not willing to follow the birthcontrol requirements as described in the informed consent or breastfeeding.

14. History or presence of obstructive pulmonary disease or urinary obstruction (contraindication for pyridostigmine).

15. This use of selective serotonin reuptake inhibitors (SSRIs).