The primary objective is to evaluate whether IGIV-C improves MG symptoms as compared to placebo in subjects with MG.

The primary objective is to evaluate the efficacy of IGIV-C in subjects with generalized myasthenia gravis (MG) on standard of care treatment at study entry in terms of improvement in MG symptoms as measured by the mean change in Quantitative Myasthenia Gravis (QMG) score from Baseline (Week 0) to Week 24 as compared to placebo.

The safety objective of this study is to evaluate the safety and tolerability of IGIV-C loading dose of 2 g/kg followed by 7 maintenance dosages of 1 g/kg every 3 weeks through Week 21 in subjects with MG.

Inclusion Criteria:

• Anti-acetylcholine receptor (AChR) antibody positive

• Confirmed diagnosis of generalized myasthenia gravis (MG).

• Myasthenia Gravis Foundation of America (MGFA) classification of Class II, III, or IVa inclusive at Screening.

• QMG >= 10 at Screening. Note: Subjects who only have a history of ocular MG may not enroll.

• Receiving standard of care MG treatment at a stable dose consisting of any one of the following for the time intervals delineated below (time intervals apply to medications and maintenance of stable dose level):

  1. Cholinesterase inhibitor (pyridostigmine or equivalent) for at least 2 weeks prior to Screening and no immunosuppressants

  2. Cholinesterase inhibitor (pyridostigmine or equivalent) for at least 2 weeks prior to Screening AND/OR only one of the following:

     1. Prednisone (up to 60 mg/day or equivalent) for at least 2 months prior to Screening, OR
     2. Azathioprine for at least 6 months prior to Screening, OR
     3. Mycophenolate mofetil for at least 6 months prior to Screening, OR
     4. Methotrexate for at least 6 months prior to Screening, OR
     5. Cyclosporine or tacrolimus for at least 3 months prior to Screening

  3. Cholinesterase inhibitor (pyridostigmine or equivalent) for at least 2 weeks prior to Screening AND/OR prednisone (up to 60 mg/day or equivalent) for at least one month prior to Screening and only one of the following:

     1. Azathioprine for at least 6 months prior to Screening, OR
     2. Mycophenolate mofetil for at least 6 months prior to Screening, OR
     3. Methotrexate for at least 6 months prior to Screening, OR
     4. Cyclosporine or tacrolimus for at least 3 months prior to Screening

Exclusion Criteria:

• Have received cyclophosphamide or any other immunosuppressive agent apart from the ones allowed per inclusion criteria within the past 6 months
• Any change in MG treatment regimen between Screening (Week -3, Visit 0) and Baseline (Week 0, Visit 1)
• Greater than two point change in QMG score, increased or decreased, between Screening (Week -3, Visit 0) and Baseline (Week 0, Visit 1)
• Any episode of myasthenic crisis in the one month prior to Screening
• Evidence of malignancy within the past 5 years (non-melanoma skin cancer, carcinoma in situ of cervix is allowed) or thymoma potentially requiring surgical intervention during the course of the trial (intent to perform thymectomy)
• Thymectomy within the preceding 6 months
• Rituximab, belimumab, eculizumab or any monoclonal antibody used for immunomodulation within the past 12 months
• Have received immune globulin (Ig) treatment given by intravenous (IV), subcutaneous, or intramuscular route within the last 3 months
• Current known hyperviscosity or hypercoagulable state
• Currently receiving anti-coagulation therapy (vitamin K antagonists, nonvitamin K antagonist oral anticoagulants [e.g., dabigatran etexilate, rivaroxaban, edoxaban, and apixaban], parenteral anticoagulants [e.g., fondaparinux]). Note that oral anti-platelet agents are allowed (e.g., aspirin, clopidogrel, ticlodipine)
• Documented diagnosis of thrombotic complications to polyclonal intravenous immunoglobulin (IVIg) therapy in the past
• History of recent (within the last year) myocardial infarction or stroke
• Uncontrolled congestive heart failure; embolism; or historically documented (within the last year) electrocardiogram (ECG) changes indicative of myocardial ischemia or atrial fibrillation
• History of chronic alcoholism or illicit drug abuse (addiction) in the 12 months preceding the Screening/Week -3 (Visit 0)
• Plasma exchange (PLEX) performed within the last 3 months
• Renal impairment (i.e., serum creatinine exceeds more than 1.5 times the upper limit of normal [ULN] for the expected normal range for the testing laboratory).
• Hemoglobin levels less than 9 g per dL