This research study is designed to test the use of ranolazine in patients with angina (chest discomfort due to reduced blood supply to the heart) due to microvascular coronary dysfunction (MCD; abnormalities in the small blood vessels of the heart). This drug is approved by the U.S. Food and Drug Administration (FDA) for treatment of chronic angina. The FDA has approved this drug based on studies primarily on patients with chronic angina with major blockages of the arteries.

This is a randomized, double-blinded, placebo- controlled, and cross-over clinical trial. 147 subjects will be enrolled at two clinical sites, with projected 9-10% dropout and anticipated 134 completed subjects. To maintain blinding of the investigators, the study randomization table will be kept in Pharmacy Service. The sponsor will ship the study drug directly to the Pharmacy Service. The pharmacy service will also be responsible for dispersing the study drug.

There are 4 study visits (2 visits in each study period) in this study. Subjects will be in this study for about 6 weeks from Week 0 - baseline visit to Week 6 - exit visit. Besides the procedure of study medication mentioned above, other study procedures include informed consent, physical exam, questionnaires, EKG for safety assessment, blood collection for laboratory testing, cardiac MRI, and follow-up events. In sum, participants will be asked to undergo 2 cardiac MRI's and fill out questionnaires 4 times. They will be asked to participate for 6 weeks with two 2-week courses (with a treatment window period of 5 days), one with ranolazine and the other with placebo (without knowing which they are taking). There is a 2-week washout period between treatments. The participants will otherwise remain on all their usual medications. The physicians will also be blinded to which medication the subject is receiving.

Participation in this study will be approximately 6 weeks, which consists of two 2-week study periods and in between a 2-week washout period:

1. During the first 2-week period: Subjects will be randomized to first receive either the ranolazine or a placebo pill (sugar pill with no active medicine). Subjects will take the extended-release ranolazine or a placebo pill for a total of 2 weeks. Subjects will take 500 mg twice daily for the first 1 week and then 1000 mg twice daily for an additional 1 week. Subjects who are unable to take the higher dose due to side effects will remain on 500 mg twice daily for the entire study period. After the 2 weeks, the participant will have a Cardiac MRI and complete study questionnaires. These tools will allow us to evaluate if the participant is doing better on the medication.
2. 2-week washout period: Subject will then be asked to go 2 weeks without any study medication (ranolazine or placebo).
3. During the second 2-week period: Subject will then be given either extended release ranolazine or placebo depending on which was received the first time for a total of 2 weeks. Subjects will take 500 mg twice daily for the first 1 week and then 1000 mg twice daily for an additional 1 week. Subjects who are unable to take the higher dose due to side effects will remain on 500 mg twice daily for entire study period. This 2-week period will again be followed by a final Cardiac MRI and questionnaire completion.

Inclusion Criteria:

1. Men or women age >18 from diverse racial/ethnic groups;
2. Competent to give informed consent;
3. Patients with chronic angina or its equivalent;
4. Coronary angiogram revealing MCD with no obstructive CAD (epicardial coronary stenosis <50% luminal diameter stenosis);
5. Left ventricular ejection fraction > or = 45%;
6. Objective evidence of ischemia by noninvasive methods such as exercise stress test, stress Echo, MRI, or SPECT;
7. Patients with 10% myocardial ischemia by Cardiac magnetic resonance imaging (CMRI) myocardial perfusion reserve index ≤ 2.0 or abnormal coronary reactivity testing (CFR < 2.5, or ACH response of no dilation or constriction, determined by local site read).

Exclusion Criteria:

1. Acute coronary syndrome (defined by WHO), cardiogenic shock or requiring inotropic or intra-aortic balloon support;
2. Planned percutaneous coronary intervention or CABG or established obstructive CAD with ischemia eligible for revascularization, acute MI;
3. Prior non-cardiac illness with an estimated life expectancy <4 years;
4. Unable to give informed consent;
5. Allergy or contra-indication to CMRI testing, including renal failure, claustrophobia, and asthma, uncontrolled moderate hypertension (sitting blood pressure >160/95mmHg with measurements recorded on at least 2 occasions), conditions likely to influence outcomes: Severe lung, creatinine >1.8 or CrCl ≤ 50ml/min) or hepatic disease;
6. Surgically uncorrected significant congenital or valvular heart disease and other disease likely to be fatal or require frequent hospitalization within the next six months;
7. Adherence or retention reasons;
8. Unwilling to complete follow-up evaluation including repeat testing, documented obstructive hypertrophic cardiomyopathy;
9. Aortic stenosis (valve area <1.5cm);
10. LV dysfunction (ejection fraction ≤35%);
11. History of significant cocaine or amphetamine abuse;
12. Taking potent CYP3A4 inhibitors (ketoconazole, itraconazole, nefazodone, troleandomycin, clarithromycin, ritonavir, nelfinavir);
13. Women who are pregnant.

Total 10 dropout (4 dropout during period 1, 4 dropout during washout, 2 dropout during period 2)

6 weeks

The SAEs are based on the publication which represent all SAEs throughout the study.