Lysine is an essential amino acid. Because it's charged, it must be imported by a protein-channel through the membrane. It can have toxic effects on cancer cells when combined with other therapies. You can check out this article "Chemists Kill Cancer Cells With Light-Activated Molecules
L-lysine ( 100 mM) has no toxic effect. Poly L-lysine is also not toxic. For monolayer cells, poly-L-lysine is used to coat the plastic surface to increase binding sites for monolayer attachment
Poly-D-lysine and Poly L-lysine are used to coat the glass cover slip surface to increase binding sites for monolayer attachment of the neurons after 2-3 times washing. This is having the toxicity due to that we wash 2 to 3 times which will help to attach the cell but not toxic to cell.