The primary outcome measure for Phase I was to determine the maximum tolerated dose (MTD) and characterize the dose limiting toxicities (DLT) of IMGN901 when administered in combination with carboplatin/etoposide chemotherapy followed by IMGN901 alone in patients with solid tumors. For the purposes of dose escalation and determination of MTD, DLTs were defined as AEs or abnormal laboratory values related to study treatment which occurred in Cycle 1 of the Dose Escalation phase, including any AEs that resulted in failure to meet the criteria for re-treatment. The following events were considered DLTs (using the most current version of CTCAE): febrile neutropenia; Grade 4 thrombocytopenia or Grade 3 thrombocytopenia with bleeding; ≥ Grade 3 peripheral neuropathy; ≥ Grade 3 vomiting, nausea, or diarrhea that persisted despite the use of optimal therapy; other ≥ grade 3 non-hematologic toxicity (with the exception of brief fatigue i.e. ≤ 72 hours and alopecia)

A total of 33 patients were analyzed as part of the Dose escalation phase.

The primary outcome measure for Phase II was to determine the efficacy of IMGN901 in combination with carboplatin/etoposide chemotherapy as first-line treatment for patients with extensive stage small cell lung cancer. PFS was defined as the time from enrollment until objective tumor progression according to RECIST 1.1 or death on study due to any cause, whichever occurred first. Only the results from the experimental arm (IMGN901 + carboplatin + etoposide) are presented as the primary objective of this phase of the study was to compare PFS in the experimental arm (triplet combination) against historical PFS rates for carboplatin and etoposide. The control arm was planned primarily to reliably assess the safety of IMGN901 and to demonstrate whether the or not the historical assumptions regarding efficacy were confirmed.

A total of 82 patients from the IMGN901 + carboplatin + etoposide safety population (N=94) were included in the efficacy analyses, due to the lack of post-baseline evaluations for 12 participants.

A primary outcome measure for Phase I was to determine the maximum tolerated dose (MTD) of IMGN901 when administered in combination with carboplatin/etoposide chemotherapy followed by IMGN901 alone in patients with solid tumors. The MTD was determined based on DLTs that occurred during Cycle 1.

During escalation, carboplatin dosing was reduced from AUC 6 to AUC 5 due to poor tolerability; therefore the MTD for IMGN901 was determined in combination with carboplatin AUC5 and 100 mg/m^2 etoposide

To assess the type and frequency of Adverse Events (AEs) and Serious Adverse Events (SAEs). An AE was defined as any noxious, pathologic, or unintended change un anatomical, physiologic, or metabolic function as indicated by physical signs, symptoms, or laboratory changes occurring in any phase of the study, whether or not deemed study drug-related. An SAE was any AE resulting in death, life-threatening experience, initial or prolonged in-patient hospitalization, persistent or significant disability/incapacity, or congenital defect. All AEs were reported from the time of the first dose of study treatment until 28 days after the final dose of study drug. the severity of AEs were graded by the Investigator using National cancer Institute (NCI) Common Terminology Criteria for AEs (CTCAE) version 4.0.

The trial was not empowered to permit a statistically informative comparison of the randomized treatment groups with respect to PFS. The activity of IMGN901 was assessed by comparing the PFS rate at 6 months in the IMGN901 experimental arm against the historical 6-month PFS rate of 0.44 (equivalently a median PFS = 5 months). Only the results from the experimental arm (IMGN901 + carboplatin + etoposide) are presented as the objective was to compare PFS at 6 months in the experimental arm (triplet combination) against the historical PFS rate of 0.44 (equivalently a median PFS = 5 months) for carboplatin and etoposide. The control arm was planned primarily to reliably assess the safety of IMGN901 and to demonstrate whether the or not the historical assumptions regarding efficacy were confirmed.

A total of 82 patients from the IMGN901 + carboplatin + etoposide safety population (N=94) were included in the efficacy analyses, due to the lack of post-baseline evaluations for 12 participants.

A secondary outcome measure for Phase II was to determine the overall survival of patients treated with IMGN901 in combination with carboplatin/etoposide chemotherapy versus carboplatin/etoposide chemotherapy alone as first-line treatment for patients with extensive stage small cell lung cancer.

A total of 121 patents were included in the analyses (82 in Arm1 and 39 in Arm 2) due to due to the lack of post-baseline evaluations for 20 participants.

OS was analyzed based on a binary definition of the number of patients dead or censored prior to 12 months and the number of patients alive at 12 months. The primary objective of this phase of the study was to compare PFS in the experimental arm (triplet combination) against historical PFS rates. The control arm was primarily planned to demonstrate whether the or not the historical assumptions regarding efficacy were confirmed. Further, the trial was not empowered to permit a statistically informative comparison of the randomized treatment groups with respect to OS and no determination of the OS rate at 12 months for the control arm was performed; therefore only the results from the experimental arm (IMGN901 + carboplatin + etoposide) are presented.

A total of 82 patients from the IMGN901 + carboplatin + etoposide safety population (N=94) were included in the efficacy analyses, due to the lack of post-baseline evaluations for 12 participants.