I just did a brief video that appears on my The Swedish hospital website/blog, addressing the question of why it is standard to give single agent but not doublet or other combinations to patients who have already received first line therapy for advanced NSCLC. As I cover in the video, the basic premise is that as treatment continues for patients on therapy for advanced NSCLC, it becomes only more and more important to balance the anticipated side effects of treatments against the incremental benefit of a more aggressive treatment.  Overall, most patients with metastatic/advanced NSCLC can tolerate and benefit meaningfully from  a combination approach for initial therapy (though a targeted therapy will be preferred for the vast majority of patients with a "driver mutation" such as an EGFR mutation or ALK rearrangement).  But beyond that point, as patients are experiencing cumulative effects from both the treatment and often the underlying cancer, the evidence reveals that combinations confer greater side effects but no accompanying significant improvement in overall survival.  

Instead, the prevailing standard of care, with the evidence to support it, is a strategy sequential single agent treatments, which may lead to further tumor shrinkage in some patients and prolonged stable disease in many others; these results often lead to an improvement in survival with a manageable balance of often modest side effects.

I welcome any questions or comments you might have.