• Lab tests (hematology, serum chemistry, and urinalysis) [ Time Frame: Screening; Dosing days 1, 8, 15, 29, 43, and 50; Follow up days 3, 8, 29, 57, and 85 ] [ Designated as safety issue: Yes ]
• Percent change in lung function: forced (expiratory) vital capacity (FVC), forced expiratory volume over one second (FEV1), total lung capacity (TLC), and carbon monoxide diffusion capacity (DLCO) [ Time Frame: Baseline, 4 weeks and 8 weeks ] [ Designated as safety issue: Yes ]
• Change in radiographic evidence of IPF as measured by high resolution computed tomography (HRCT) [ Time Frame: Baseline, week 8 ] [ Designated as safety issue: Yes ]
• Change in biomarkers isolated from bronchoalveolar lavage (BAL) [ Time Frame: Baseline, week 8 ] [ Designated as safety issue: Yes ]
• Incidence of antibodies to STX-100 [ Time Frame: Baseline, follow up days 29 and 85 ] [ Designated as safety issue: Yes ]
• Serum half-life of STX-100 [ Time Frame: Following the final dose of STX 100 at day 50 through day 85 ] [ Designated as safety issue: Yes ]

This is a multi-center, randomized, double-blind, placebo-controlled, multiple dose, dose-escalation study designed to evaluate the safety, tolerability, pharmacokinetics, immunogenicity, and impact on BAL and peripheral blood biomarkers of STX-100 in patients with IPF. Approximately 32 patients will be enrolled into 4 sequential ascending dose cohorts. Each cohort will include 8 patients randomized to receive either STX-100 (6 patients) or placebo (2 patients). Additional patients may be enrolled if deemed appropriate by the Data Safety Monitoring Board (DSMB).

• Drug: STX-100
  Subcutaneous (SC) delivery of 8 consecutive weekly doses
• Drug: Placebo
  Subcutaneous (SC) delivery of 8 consecutive weekly doses

• Experimental: STX-100
  Active treatment with study drug
  Intervention: Drug: STX-100
• Placebo Comparator: Placebo
  Intervention: Drug: Placebo

Inclusion Criteria:

1. Consenting male or female patients, 50 to 84 years old, inclusive. Patients 18 to 49 years of age are eligible if they have a histopathologic diagnosis of usual interstitial pneumonia (UIP) based upon a surgical lung biopsy in the appropriate clinical setting, and meet all other inclusion/exclusion criteria.
2. Clinical features consistent with IPF prior to screening (based on the ATS/ERS/JRS/ALAT consensus criteria for the diagnosis of IPF).
3. FVC ≥ 50% of predicted value.
4. DLco (corrected for hemoglobin) ≥ 35% predicted value.
5. Oxygen saturation > 90% by pulse oximetry while breathing ambient air at rest.
6. Residual volume ≤ 120% predicted value.
7. Ratio of FEV1 to FVC ≥ 0.65 after the use of a bronchodilator.
8. Other known causes of interstitial lung disease have been excluded (e.g., drug toxicities, environmental exposures, connective tissue diseases).
9. HRCT image fulfills the criteria for 'UIP pattern'.
10. If the HRCT image does not fulfill the criteria for 'UIP pattern' a surgical lung biopsy is necessary for the diagnosis of IPF.
11. Adequate bone marrow and liver function.
12. Patient has a life expectancy of at least 12 months.

Exclusion Criteria:

1. Findings that are diagnostic of a condition other than UIP on surgical lung biopsy, HRCT imaging, transbronchial lung biopsy, or bronchoalveolar lavage (BAL).
2. Currently receiving high dose corticosteroid, cytotoxic (e.g., chlorambucil, azathioprine, cyclophosphamide, methotrexate), anti-fibrotic (e.g., pirfenidone), vasodilator therapy for pulmonary hypertension (e.g., bosentan), unapproved (e.g., INF-γ, penicillamine, cyclosporine, mycophenolate, Nacetylcysteine), and/or investigational therapy for IPF or administration of such therapeutics within 4 weeks of initial screening. A current dose of ≤ 15 mg/day of prednisone or its equivalent is acceptable if it is anticipated the dose will remain stable during the study.
3. History of malignancy, including carcinoma during the preceding 5 years.
4. Significant cardiac disease (e.g., New York Heart Association [NYHA] Class 3 or 4; myocardial infarction within the past 6 months; unstable angina; coronary angioplasty or coronary artery bypass graft [CABG] within the past 6 months; or uncontrolled atrial or ventricular cardiac arrhythmias).
5. Serious local infection or systemic infection within 3 months prior to screening.
6. Positive test for HBsAg, HCV antibody, or HIV antibody at screening.
7. Treatment with another investigational drug, investigational device, or approved therapy for investigational use within 4 weeks of initial screening.
8. Currently listed and/or anticipated to be listed for lung transplantation within the next 6 months.