Time to progression (TTP) is defined as the interval between the date of randomization and the earliest date of progression of disease (PD) or death due to breast cancer. The investigator assessed PD based on radiological PD (imaging data) and clinical symptomatic progress (Response Evaluation Criteria in Solid Tumors [RECIST] Criteria: target lesion (TL), at least a 20% increase in the sum of largest diameter (LD) of TLs or the appearance of one or more new lesions; non-TL (NTL), the appearance of one or more new lesions and/or unequivocal progression of existing NTLs). TTP was assessed in participants who died due to breast cancer or progressed, as assessed by the investigator, as well as in those who were censored and completed follow-up and those who were censored but are still being followed. For censored participants (those without a documented date of disease progression/death due to breast cancer), the date of the last radiographic assessment was used.

Intent-to-Treat (ITT) Population: all randomized participants who had received at least one dose of randomized therapy (lapatinib or placebo)

Time to progression is defined as the interval between the date of randomization and the earliest date of progression of disease (PD) or death due to breast cancer. The IRC assessed PD based on radiological PD (imaging data) and clinical symptomatic progress (Response Evaluation Criteria in Solid Tumors [RECIST] Criteria: target lesion (TL), at least a 20% increase in the sum of largest diameter (LD) of TLs or the appearance of one or more new lesions; non-TL (NTL), the appearance of one or more new lesions and/or unequivocal progression of existing NTLs). TTP was assessed in participants who died due to breast cancer or progressed, as assessed by the independent reviewer, as well as in those who were censored and completed follow-up and those who were censored but are still being followed. For censored participants (those without a documented date of disease progression/death due to breast cancer), the date of the last radiographic assessment was used.

ITT Population

The percentage of participants with tumor response is defined as those participants with measurable disease who achieved either a complete response (CR) or partial response (PR). The Response Evaluation Criteria in Solid Tumors (RECIST) was used to evaluate the measurability of tumor lesions, to determine target lesion (TLs) and non-target lesion (NTLs). CR (TLs and NTLs): the disappearance of all TLs and NTLs; PR (for TLs): at least a 30% decrease in the sum of the largest diameter (LD) of TLs, taking as a reference the Baseline sum LD; PR (for NTLs): persistence of one or more lesions.

ITT Population

The percentage of participants with tumor response is defined as those participants with measurable disease who achieved either a complete response (CR) or partial response (PR). The RECIST criteria was used to evaluate the measurability of tumor lesions, to determine target lesion (TLs) and non-target lesion (NTLs). CR (TLs and NTLs): the disappearance of all TLs and NTLs; PR (for TLs): at least a 30% decrease in the sum of the largest diameter (LD) of TLs, taking as a reference the Baseline sum LD; PR (for NTLs): persistence of one or more lesions.

ITT Population

Percentage of participants. with CB is defined as the percentage of participants with evidence of CR (disappearance of all TLs and NTLs), PR (TLs: a >=30% decrease in the sum of the LD, taking as a reference the Baseline sum LD; NTLs: persistence of >=1 lesion), or stable disease (neither sufficient shrinkage to qualify for PR nor sufficient increase to qualify for PD) for >=6 months based on RECIST criteria. PD for TL: a >=20% increase in the sum of the LD of TLs or the appearance of >=1 new lesion. PD for NTLs: the appearance of >=1 new lesion and/or unequivocal progression of existing NTLs.

ITT Population

Time to response (TTR) is defined as the time from randomization until the first documented evidence of CR (disappearance of all TLs and NTLs) or PR (for TLs: a >=30% decrease in the sum of the LD of TLs, taking as a reference the Baseline sum LD; for NTLs: the persistence of >=1 lesion) (whichever status was recorded first). TTR data are displayed as the number of participants achieving a CR or PR by the indicated week. The investigator evaluated the TTR, and the analysis was based on responses confirmed at a repeat assessment, with the TTR taken as the first time the response was observed.

ITT Population

The investigator evaluated the DOR for the subset of participants who showed a CR (disappearance of all TLs and NTLs) or PR (TLs: a >=30% decrease in the sum of the LD of TLs, taking as a reference the Baseline sum LD; NTLs: persistence of >=1 lesion). DOR is defined as the time from the first documented evidence of PR or CR until the first documented sign of PD (TL: a >=20% increase in the sum of the LD of TLs or the appearance of >=1 new lesion; NTL: the appearance of >=1 new lesion and/or unequivocal progression of existing NTLs) or death due to breast cancer, if sooner.

ITT Population. Only participants who had a CR or PR were evaluated.

PFS is defined as the interval between the date of randomization and the earliest date of progression disease (PD) or death due to any cause, if sooner. For TLs, progressive disease is defined as at least a 20% increase in the sum of the LD of TLs or the appearance of 1 or more new lesions. For NTLs, progressive disease is defined as the appearance of 1 or more new lesions and/or unequivocal progression of existing NTLs. par., participants.

ITT Population. PFS was assessed in par. who died or progressed, as well as in those who were censored and completed follow-up and those who were censored but are still being followed. For censored par. (those without a documented date of disease progression/death due to any cause), the date of the last radiographic assessment was used.

PFS is defined as the interval between the date of randomization and the earliest date of disease progression or death due to any cause, if sooner. Six months PFS is defined as PFS at six months from the time of randomization. Raw data for 6 months PFS are not available; thus, data are presented as the number of participants who progressed or died at or prior to 6 months. For TLs, progressive disease is defined asat least a 20% increase in the sum of the LD of TLs or the appearance of 1 or more new lesions. For NTLs, progressive disease is defined as the appearance of 1 or more new lesions and/or unequivocal progression of existing NTLs. PFS was assessed in participants who died or progressed, as well as in those who were censored and completed follow-up and those who were censored but are still being followed. For censored participants (those without a documented date of disease progression/death due to breast cancer), the date of the last radiographic assessment was used.

ITT Population

Overall survival is defined as the time from randomization until death due to any cause.

ITT population. Overall survival was assessed in participants who died as well as in those who were censored and completed follow-up and those who were censored but are still being followed. For censored participants (those still alive), the date of the last contact was used.

The FACT-B questionnaire was designed to measure multidimensional quality of life (QOL) in participants with breast cancer. The physical and functional well-being subscale scores range from 0 to 28, based on 7 questions (each scored from 0 [not at all] to 4 [very much] for all subscales); the emotional and social/family well-being (1 question optional) subscale scores range from 0 to 24 (based on 6 questions), and the additional concerns subscale score ranges from 0 to 40, based on 10 questions. The FACT-B Total Score (0 [better QOL] to 144 [worse QOL]) is the sum of the subscale scores.

ITT Population. Only participants contributing data at the indicated time points were analyzed. Only observed data were collected, and score analyses were conducted using the last observation carried forward (LOCF) method: the last available on-therapy observation for a participant was used to estimate missing data points.

The FACT-G questionnaire was designed to measure multidimensional QOL in participants with cancer and includes subscales for physical, social/family, emotional, and functional well-being. The physical, social/family, and functional well-being subscale scores range from 0 to 28, based on responses to 7 questions (each question scored from 0 [not at all] to 4 [very much]); the emotional well-being subscale score ranges from 0 to 24, based on responses to 6 questions. The FACT-G Total Score (ranging from 0 [better QOL] to 108 [worse QOL]) is the sum of the subscale scores.

ITT Population. Only participants contributing data at the indicated time points were analyzed. Only observed data were collected, and score analyses were conducted using the LOCF method.

The TOI questionnaire was designed to measure multidimensional QOL in participants with cancer and includes subscales for physical, functional well-being, and additional cancer concerns. The physical and functional well-being subscale scores range from 0 to 28, based on 7 questions (each question scored from 0 [not at all] to 4 [very much]); the breast cancer unweighted subscale scores range from 0 to 36, based on 9 questions. The total TOI score (ranging from 0 [better QOL] to 92 [worse QOL]) is the sum of the TOI subscale scores.

ITT Population. Only participants contributing data at the indicated time points were analyzed. Only observed data were collected, and score analyses were conducted using the LOCF method.

The Press Laboratory collected tumor tissues of participants for ErbB2 testing. ErbB2 testing is done to detect breast cancer and predict its likely outcome. All samples were analyzed by the Press Laboratory. Participants were categorized as ErbB2 positive (overexpression of the ErbB2 gene), ErbB2 negative, and assay not done (which included participants with no available samples and those with inconclusive results). ErbB2 status is determined by immunohistochemistry (ICH) assay and fluorescence in situ hybridization (FISH) testing. Negative ErbB2 status is defined as 0 or 1+ by IHC, or as 2+ by IHC and FISH.

ITT Population

The Press Laboratory collected tumor tissues of participants for biomarker testing. All samples were analyzed by the Press Laboratory. The ratio of ErbB2 gene signals to chromosome 17 signals, which indicates the progression of breast cancer, was calculated. Low levels of amplification (few copies) may have a ratio of 2-5, whereas high levels of amplification may have a ratio >10.

ITT Population. Only participants contributing data at the indicated time points were analyzed. Only observed data were collected, and score analyses were conducted using the LOCF method.

The Press Laboratory tested tumor tissue samples (taken at Screening, prior to randomization to study treatment) to determine intra-tumoral expression levels of ErbB1, ErbB2, and other analytes associated with these pathways by IHC, the process of detecting antigens (e.g., proteins) in cells of a tissue section. The IHC assessment is expressed as: 0, no staining (no cancer cells); 1+, faint staining; 2+, weak to moderate complete staining; 3+, strong complete staining (many cancer cells). A status of "Assay not done" was assigned to participants with no available samples and to those with inconclusive results. If strong staining is observed, breast cancer that has high levels of HER2 expression (overexpression) is indicated. If moderate/weak staining is observed (IHC=2+), breast cancer that has low/moderate expression levels is indicated. When no staining is observed (IHC=0), breast cancer HER2 expression may be below the level of detection of the assay.

ITT Population

The Press Laboratory tested participants who were 2+ (weak to moderate complete staining) or 3+ (strong complete staining) for ErbB2 overexpression by IHC for ErbB2 gene amplification using the FISH assay. The results of the FISH assay can be ErbB2 gene "amplification" (increased number of copies of the ErbB2 gene) or "non-amplification" (not many copies of the ErbB2 gene). A status of "Assay not done" was assigned to those participants with no available samples and to those with inconclusive results (e.g., due to hybridization or staining problems).

ITT Population

The Quest Laboratory collected blood samples for quantitative determination of serum ErbB1. The results of serum monitoring were used to compare tumor response rates following randomized therapy.

ITT Population. Only participants contributing data at the indicated time points were analyzed. Only observed data were collected, and score analyses were conducted using the LOCF method.

The Quest Laboratory collected blood samples for quantitative determination of serum ErbB2. The results of serum monitoring were used to compare tumor response rates following randomized therapy.

ITT Population. Only participants contributing data at the indicated time points were analyzed. Only observed data were collected, and score analyses were conducted using the LOCF method.

The severity of adverse events was graded per the National Cancer Institute-Common Terminology Criteria for Adverse Events (NCI-CTCAE), Version 3. Grades 1 through 5 have unique clinical descriptions of severity for each AE based on the following general guideline: Grade 1, Mild AE; Grade 2, Moderate AE; Grade 3, Severe AE; Grade 4, Life-threatening or disabling AE; Grade 5, death related to AE.

Safety Population: all randomized participants who received at least one dose of investigational product (based on the actual treatment received if this differed from that to which the participant was randomized). Two participants randomized to the placebo group actually received lapatinib.