It is difficult to treat lung metastasis with systemic chemotherapy.

Why?

Obstructive metastatic foci in lung arterioles decrease the pulmonary vascular network [1].

It is necessary to develop a new drug delivery method instead of systemic chemotherapy.

Compared with normal lungs, the vascular network in metastatic lungs is drastically reduced.
Unless the obstructed vessels are overcome, the response rate to systemic chemotherapy will not improve.

Reference
[1] A. Sukhbaatar, M. Sakamoto, S. Mori, T. Kodama, Analysis of tumor vascularization in a mouse model of metastatic lung cancer, Scientific Reports, 9 (2019) 16029.