Inclusion Criteria:

• Male or female subjects ≥18 years of age, at screening Visit 1:
  • If potential subjects are >75 years of age, discussion between the investigator and the Medical Monitor must take place;
• Body mass index ≤40 kg/m2, at screening Visit 1;
• A history of nephrotic syndrome due to iMN as confirmed by documented results from a renal biopsy performed within 4 years prior to screening Visit 1:
  • If a biopsy has been performed between 4-8 years prior to screening, and if the subject has no signs or symptoms of diabetes or other clinical diagnoses that could suggest a change in renal histology in the opinion of the investigator and the Medical Monitor, the subject is eligible.
• Renal target disease requirements:
  • Total urine protein of ≥3.0g (≥3000mg) from the 24-hour urine returned at Visit 1A, AND.
  • An estimated glomerular filtration rate (eGFR) value >25mL/min/1.73m2 at Visit 1A (as calculated using the abbreviated Modification of Diet in Renal Disease [MDRD] equation.
• Any prior course of at least 1 month of treatment with ≥1 of an immunosuppressant therapy(ies) for iMN:
  • Subjects must be followed for at least 3 months after treatment prior to screening with the exception of rituximab or a cytotoxic based therapy, where the follow-up period is 6 months after treatment. If after follow-up it was determined that the subject did not achieve a complete or partial remission or suffered a relapse after achieving a partial remission, the subject will be eligible for the study.
  • If in the investigator's opinion, the subject should be enrolled prior to meeting the follow-up period criteria and the decrease in proteinuria is no longer occurring, discussion between the investigator and the Medical Monitor must take place for approval to enter screening.
• History of treatment-resistant iMN defined as either having had no remission or having suffered a relapse after achieving a partial remission to their most recent standard treatment regimen as defined in the Definition of Response Status Table despite treatment with at least 1 month of treatment with a prior therapy for iMN. Note the following:
  • If the subject has been treated with prior standard therapy and can no longer be re-treated with any component of that therapy, regardless of whether a complete or partial remission was achieved, then the subject may be eligible, but approval from the Medical Monitor is required.
    • For example, if early discontinuation of standard therapy occurred because of a serious adverse event (Grade 3 or 4) during the treatment, regardless of whether a partial or complete remission was achieved, then the subject may be eligible.
  • If (a) does not apply, and the subject did not have either a partial or complete remission to the most recent treatment regimen, then the subject is eligible.
  • If (a) does not apply, and the subject achieved a partial remission from the most recent treatment regimen, and later relapse occurred, then the subject is eligible.
• Antihypertensive treatment including use of ACE inhibitors and/or ARB:
  • Unless there is a history of intolerance to ACE inhibitors or ARB therapy, the subject must be treated with at least of these agents.
  • Treatment with ACE inhibitor and/or ARB for ≥3 months prior to screening Visit 1A, with stable maintenance dose for ≥30 days prior to randomization.
  • If treated with other antihypertensive therapies, treatment duration of ≥30 days and stable maintenance dose for ≥7 days prior to screening Visit 1A.
• Blood pressure determined by the average of ≥3 seated readings taken ≥5 minutes apart during the screening period at Visit 1A:
  • Mean systolic blood pressure ≤140 mmHg and
  • Mean diastolic blood pressure ≤80 mmHg.

Exclusion Criteria:

• Therapies and/or medications:
  • History of previous use of Acthar for treatment of nephrotic syndrome;
  • Prior sensitivity to Acthar or other porcine protein products; or
  • Planned treatment with live or live attenuated vaccines once enrolled in the study.
• Contraindication to Acthar per Prescribing Information: scleroderma, osteoporosis, systemic fungal infections, ocular herpes simplex, recent surgery, history of or the presence of peptic ulcer, congestive heart failure, uncontrolled hypertension, primary adrenocortical insufficiency, or adrenocortical hyperfunction.
  • For the purpose of this study: "history" of peptic ulcer is defined as ≤6 months prior to Visit 1A.
• Renal target disease exclusions:
  • Subjects with known diabetic nephropathy or nephrotic syndrome due to a disease or process other than idiopathic membranous nephropathy, or
  • Subjects requiring diagnostic or interventional procedure requiring a contrast agent must delay screening/randomization for at least 7 days.
• History of Systemic Lupus Erythematosus.
• Type 1 or Type 2 diabetes mellitus (prior diagnosis of gestational diabetes mellitus is not an exclusion).
• History of Deep Vein Thrombosis (DVT) ≤6 months prior to screening Visit 1A.
• Presence of renal vein thrombosis:
  • Known current diagnosis by ultrasound, magnetic resonance imaging (MRI) or computed tomography scan;
  • Signs or symptoms consistent with occurrence of acute renal vein thrombosis (hematuria in combination with flank pain and >30% unexplained acute rise in serum creatinine) with renal vein thrombosis confirmed by ultrasound, MRI or computed tomography scan.
• Cardiovascular exclusions:
  • History of or active congestive heart failure (NYHA Functional Classification of CHF Class II through Class IV), or.
  • History of known dilated cardiomyopathy with left ventricular ejection fraction ≤40%, or.
  • Occurrence of any of the following within 3 months of screening Visit 1A:
    • Unstable angina.
    • Myocardial infarction.
    • Coronary artery bypass graft or percutaneous transluminal coronary angioplasty.
    • Transient ischemic attack or cerebrovascular disease.
    • unstable arrhythmia.