Spleen volume was measured using magnetic resonance imaging (MRI) or CT scan in participants who were not candidates for MRI or when MRI was not readily available. The MRIs or CTs were read in the central imaging laboratory. Spleen volume was obtained by outlining the circumference of the organ and determining the volume using the technique of least squares. The same method (MRI or CT) was used for a given participant unless a new contraindication to the use of MRI (eg, pacemaker insertion) occurred. A positive value indicates an increase in spleen volume and a negative value indicates a decrease in spleen volume.

Full Analysis Set included all enrolled participants who received at least 1 dose of itacitinib. Here, Overall Number of participants analyzed ("N") signifies participants who were evaluable for this outcome measure.

Spleen volume was measured using MRI or CT scan in participants who were not candidates for MRI or when MRI was not readily available. The MRIs or CTs were read in the central imaging laboratory. Spleen volume was obtained by outlining the circumference of the organ and determining the volume using the technique of least squares. The same method (MRI or CT) was used for a given participant unless a new contraindication to the use of MRI (eg, pacemaker insertion) occurred. A positive value indicates an increase in spleen volume and a negative value indicates a decrease in spleen volume.

Full Analysis Set included all enrolled participants who received at least 1 dose of itacitinib. Here, "N" signifies participants who were evaluable for this outcome measure.

An AE is defined as any untoward medical occurrence associated with the use of a drug in humans, whether considered drug related, that occurs after a participant provides informed consent. A TEAE is any AE either reported for the first time or worsening of a pre-existing event after first dose of study drug. An SAE is an AE resulting in: death; initial/prolonged inpatient hospitalization; life-threatening; persistent or significant disability/incapacity; congenital anomaly.

Safety Population included all enrolled participants who received at least 1 dose of itacitinib.

Laboratory investigation included hematology, clinical chemistry, coagulation and urinalysis. Clinical significance was determined by the investigator. The number of participants with clinically significant changes from baseline in laboratory parameters were reported.

Safety Population included all enrolled participants who received at least 1 dose of itacitinib.

Vital signs included body temperature, systolic and diastolic blood pressure, pulse rate, respiratory rate, weight and height. Clinical significance was determined by the investigator. The number of participants with clinically significant changes from baseline in vital signs were reported.

Safety Population included all enrolled participants who received at least 1 dose of itacitinib.

Spleen volume was measured using magnetic resonance imaging (or CT scan in applicable participants). MRI of the upper and lower abdomen and pelvis was performed, to assess spleen volumes. MRI was performed with a body coil. The MRIs were read in the central imaging laboratory. Spleen volume was obtained by outlining the circumference of the organ and determining the volume using the technique of least squares. MRI was the preferred method for obtaining spleen volume data. The CT scans were processed by the same central laboratory used for MRIs. The same method (MRI or CT) was used for a given participant unless a new contraindication to the use of MRI (eg, pacemaker insertion) occurred.

Full Analysis Set included all enrolled participants who received at least 1 dose of itacitinib. Here, "N" signifies participants who were evaluable for this outcome measure.

Spleen volume was measured using magnetic resonance imaging (or CT scan in applicable participants). MRI of the upper and lower abdomen and pelvis was performed, to assess spleen volumes. MRI was performed with a body coil. The MRIs were read in the central imaging laboratory. Spleen volume was obtained by outlining the circumference of the organ and determining the volume using the technique of least squares. MRI was the preferred method for obtaining spleen volume data. The CT scans were processed by the same central laboratory used for MRIs. The same method (MRI or CT) was used for a given participant unless a new contraindication to the use of MRI (eg, pacemaker insertion) occurred.

Full Analysis Set included all enrolled participants who received at least 1 dose of itacitinib. Here, "N" signifies participants who were evaluable for this outcome measure.

Measurement of spleen length below the left costal margin was measured by palpation. Spleen size was determined at every physical examination with the participant in the recumbent (not left decubitus) position. The edge of the spleen was determined by palpation, and measured in centimeters, using a soft ruler, from the costal margin to the point of greatest splenic protrusion. The measurements should be noted and the site at which it was determined listed (eg, anterior axillary line, midclavicular line, and/or subxiphoid). A positive value indicates an increase in spleen volume and a negative value indicates a decrease in spleen volume.

Full Analysis Set included all enrolled participants who received at least 1 dose of itacitinib. Here, "N "signifies number of participants analyzed for this outcome measure and "n" signifies number of participants with data available at a particular time point.

Measurement of spleen length below the left costal margin was measured by palpation. Spleen size was determined at every physical examination with the participant in the recumbent (not left decubitus) position. The edge of the spleen was determined by palpation, and measured in centimeters, using a soft ruler, from the costal margin to the point of greatest splenic protrusion. The measurements should be noted and the site at which it was determined listed (eg, anterior axillary line, midclavicular line, and/or subxiphoid). A positive value indicates an increase in spleen volume and a negative value indicates a decrease in spleen volume.

Full Analysis Set included all enrolled participants who received at least 1 dose of itacitinib. Here, "N" signifies number of participants analyzed for this outcome measure and "n" signifies number of participants with data available at a particular time point.

Symptoms of myelofibrosis were assessed using a modified MFSAF Version 2.0 diary. Using the diary, participants rated the following symptoms on a scale from 0 (absent/as good as it can be) to 10 (worst imaginable/as bad as it can be): itching, night sweats, abdominal discomfort/bloating, early satiety, pain under the ribs on left side and bone/muscle pain. The total symptom score ranged from 0-60 and was calculated as the sum of the 6 symptom scores. A higher score indicates worse symptoms.

Full Analysis Set included all enrolled participants who received at least 1 dose of itacitinib. Here, "N" signifies number of participants analyzed for this outcome measure and "n" signifies number of participants with data available at a particular time point.

Symptoms of myelofibrosis were assessed using a modified MFSAF Version 2.0 diary. Using the diary, participants rated the following symptoms on a scale from 0 (absent/as good as it can be) to 10 (worst imaginable/as bad as it can be): itching, night sweats, abdominal discomfort/bloating, early satiety, pain under the ribs on left side and bone/muscle pain. The total symptom score ranged from 0-60 and was calculated as the sum of the 6 symptom scores. A higher score indicates worse symptoms.

Full Analysis Set included all enrolled participants who received at least 1 dose of itacitinib. Here, "N" signifies number of participants analyzed for this outcome measure and "n" signifies number of participants with data available at a particular time point.

Symptoms are evaluated by the MPN-SAF TSS. The MPN-SAF TSS assessed by the participants themselves and this includes fatigue, concentration, early satiety, inactivity, night sweats, itching, bone pain, abdominal discomfort, weight loss, and fevers. Scoring is from 0 (absent/as good as it can be) to 10 (worst imaginable/as bad as it can be) for each item. The MPN-SAF TSS is the summation of all the individual scores (0-100 scale). A higher score indicates worse symptoms.

Full Analysis Set included all enrolled participants who received at least 1 dose of itacitinib. . Here, "N" signifies number of participants analyzed for this outcome measure and "n" signifies number of participants with data available at a particular time point.

Symptoms are evaluated by the MPN-SAF TSS. The MPN-SAF TSS assessed by the participants themselves and this includes fatigue, concentration, early satiety, inactivity, night sweats, itching, bone pain, abdominal discomfort, weight loss, and fevers. Scoring is from 0 (absent/as good as it can be) to 10 (worst imaginable/as bad as it can be) for each item. The MPN-SAF TSS is the summation of all the individual scores (0-100 scale). A higher score indicates worse symptoms. Note that the mean percentage change can vary in direction from the mean absolute change because percent increases (but not decreases) can exceed 100%.

Full Analysis Set included all enrolled participants who received at least 1 dose of itacitinib. Here, "N" signifies number of participants analyzed for this outcome measure and "n" signifies number of participants with data available at a particular time point.

Symptoms of myelofibrosis were assessed using the PGIC questionnaire. Using the questionnaire, participants rated the overall sense of treatment effect on their symptoms on a scale of 1 (very much improved)- 7(very much worse). The specific wording was: Since the start of the treatment you have received in this study, your myelofibrosis symptoms are: 1) Very much improved, 2) Much improved, 3) Minimally improved, 4) No change, 5) Minimally worse, 6) Much worse, 7) Very much worse. A higher score indicates worse symptoms.

Full Analysis Set included all enrolled participants who received at least 1 dose of itacitinib. Here, "N" signifies number of participants analyzed for this outcome measure and "n" signifies number of participants with data available at a particular time point.

Treatment response (complete remission [CR] or partial remission [PR]) graded per IWG-MRT. CR: Bone marrow (BM): < 5% blasts; ≤ Grade 1 MF, Peripheral blood: Hemoglobin (Hb) ≥ 100 grams per liter (g/L), < upper normal limit (UNL); neutrophil count ≥ 1 × 10^9/L and < UNL; Platelet count ≥ 100 × 10^9/L and < UNL; < 2% immature myeloid cells (IMCs) and Clinical: Resolution of disease symptoms; spleen, liver not palpable; no evidence of extramedullary hematopoeisis (EMH). PR: Peripheral blood: Hb ≥ 100 g/L and < UNL; neutrophil count ≥ 1 × 10^9/L and < UNL; platelet count ≥ 100 × 10^9/L and < UNL; < 2% IMCs and Clinical: Resolution of symptoms; spleen and liver not palpable; no evidence of EMH or BM: < 5% blasts; ≤ Grade 1 MF; and peripheral blood: Hb≥ 85 g/L but < 100 g/L and < UNL; neutrophil count ≥ 1 × 10^9/L and < UNL; platelet count ≥ 50 × 10^9/L but < 100 × 10^9/L and < UNL; < 2% IMCs and Clinical: Resolution of symptoms; spleen, liver not palpable; no evidence of EMH.

Full Analysis Set included all enrolled participants who received at least 1 dose of itacitinib.

AUCtau defined as area under the concentration-time curve over a dosing interval for Itacitinib. The concentrations of itacitinib in plasma were determined using a validated Liquid Chromatography with tandem mass spectrometry (LC/MS/MS) method with an assay range of 5 to 5000 nM. The PK parameters were calculated using standard noncompartmental analysis in Phoenix WinNonlin® v8.2 (Certara USA Inc., Princeton, NJ). Itacitinib PK data for Cohort A on Week 2 were not available as itacitinib was to be held until the completion of PK sample collection.

Pharmacokinetic evaluable population included all participants who received at least 1 dose of itacitinib and provided at least 1 post-dose plasma sample for PK. Here, "N " signifies number of participants analyzed for this outcome measure.

AUCtau defined as area under the concentration-time curve over a dosing interval for ruxolitinib. The concentrations of ruxolitinib in plasma were determined using a validated LC/MS/MS method with an assay range of 1 to 1000 nM. The PK parameters were calculated using standard noncompartmental analysis in Phoenix WinNonlin® v8.2 (Certara USA Inc., Princeton, NJ).

Pharmacokinetic evaluable population included all participants who received at least 1 dose of ruxolitinib and provided at least 1 post-dose plasma sample for PK. Here, "N " signifies number of participants analyzed for this outcome measure. Only the data from 15mg QD is shown as it is the only dose level with at least three participants at both week 2 and week 4.

Clearance of a drug was measure of the rate at which a drug is metabolized or eliminated by normal biological processes. The concentrations of itacitinib in plasma were determined using a validated LC/MS/MS method with an assay range of 5 to 5000 nM. The PK parameters were calculated using standard noncompartmental analysis in Phoenix WinNonlin® v8.2 (Certara USA Inc., Princeton, NJ). Data for Cohort A (Itacitinib) on Week 2 was not available as Cohort A, itacitinib was to be held on Week 2 until the completion of the PK sample collection.

Pharmacokinetic evaluable population included all participants who received at least 1 dose of itacitinib and provided at least 1 post-dose plasma sample for PK. Here, "N " signifies number of participants analyzed for this outcome measure.

Clearance of a drug was measure of the rate at which a drug is metabolized or eliminated by normal biological processes. The concentrations of ruxolitinib in plasma were determined using a validated LC/MS/MS method with an assay range of 1 to 1000 nM. The PK parameters were calculated using standard noncompartmental analysis in Phoenix WinNonlin® v8.2 (Certara USA Inc., Princeton, NJ).

Pharmacokinetic evaluable population included all participants who received at least 1 dose of ruxolitinib and provided at least 1 post-dose plasma sample for PK. Here, "N " signifies number of participants analyzed for this outcome measure. Only the data from 15mg QD is shown as it is the only dose level with at least three participants at both week 2 and week 4.

The concentrations of itacitinib in plasma were determined using a validated LC/MS/MS method with an assay range of 5 to 5000 nM. The PK parameters were calculated using standard noncompartmental analysis in Phoenix WinNonlin® v8.2 (Certara USA Inc., Princeton, NJ). Data for Cohort A (Itacitinib) on Week 2 was not available as Cohort A, itacitinib was to be held on Week 2 until the completion of the PK sample collection.

Pharmacokinetic evaluable population included all participants who received at least 1 dose of itacitinib and provided at least 1 post-dose plasma sample for PK. Here, "N " signifies number of participants analyzed for this outcome measure.

The concentrations of ruxolitinib in plasma were determined using a validated LC/MS/MS method with an assay range of 1 to 1000 nM. The PK parameters were calculated using standard noncompartmental analysis in Phoenix WinNonlin® v8.2 (Certara USA Inc., Princeton, NJ).

Pharmacokinetic evaluable population included all participants who received at least 1 dose of ruxolitinib and provided at least 1 post-dose plasma sample for PK. Here, "N " signifies number of participants analyzed for this outcome measure. Only the data from 15mg QD is shown as it is the only dose level with at least three participants at both week 2 and week 4.

The concentrations of itacitinib in plasma were determined using a validated LC/MS/MS method with an assay range of 5 to 5000 nM. The PK parameters were calculated using standard noncompartmental analysis in Phoenix WinNonlin® v8.2 (Certara USA Inc., Princeton, NJ). Data for Cohort A (Itacitinib) on Week 2 was not available as Cohort A, itacitinib was to be held on Week 2 until the completion of the PK sample collection.

Pharmacokinetic evaluable population included all participants who received at least 1 dose of itacitinib and provided at least 1 post-dose plasma sample for PK. Here, "N " signifies number of participants analyzed for this outcome measure.

The concentrations of ruxolitinib in plasma were determined using a validated LC/MS/MS method with an assay range of 1 to 1000 nM. The PK parameters were calculated using standard noncompartmental analysis in Phoenix WinNonlin® v8.2 (Certara USA Inc., Princeton, NJ).

Pharmacokinetic evaluable population included all participants who received at least 1 dose of ruxolitinib and provided at least 1 post-dose plasma sample for PK. Here, "N " signifies number of participants analyzed for this outcome measure. Only the data from 15mg QD is shown as it is the only dose level with at least three participants at both week 2 and week 4.

Ctau is defined as concentration at the end of the dosing interval of ruxolitinib.The concentrations of itacitinib in plasma were determined using a validated LC/MS/MS method with an assay range of 5 to 5000 nM. The PK parameters were calculated using standard noncompartmental analysis in Phoenix WinNonlin® v8.2 (Certara USA Inc., Princeton, NJ). Data for Cohort A (Itacitinib) on Week 2 was not available as Cohort A, itacitinib was to be held on Week 2 until the completion of the PK sample collection.

Pharmacokinetic evaluable population included all participants who received at least 1 dose of itacitinib and provided at least 1 post-dose plasma sample for PK. Here, "N " signifies number of participants analyzed for this outcome measure.

Ctau is defined as concentration at the end of the dosing interval of ruxolitinib. The concentrations of ruxolitinib in plasma were determined using a validated LC/MS/MS method with an assay range of 1 to 1000 nM. The PK parameters were calculated using standard noncompartmental analysis in Phoenix WinNonlin® v8.2 (Certara USA Inc., Princeton, NJ).

Pharmacokinetic evaluable population included all participants who received at least 1 dose of ruxolitinib and provided at least 1 post-dose plasma sample for PK. Here, "N " signifies number of participants analyzed for this outcome measure. Only the data from 15mg QD is shown as it is the only dose level with at least three participants at both week 2 and week 4.