How do I avoid precipitation in a 3.6% cysteine. HCl Eye drops?

Cysteine. HCl gives a very acidic, unacceptable pH in water. Setting a more neutral pH accelerated the oxidation of cysteine to cystine, which is very little soluble in water and consequently precipitates within 24 hours. In the literature, the use of penicillamine is known for the treatment of Cystinuria in which cystine may slowly enter cysteine in solution after ingestion of penicillamine. Given the similar pharmacological action of cystine, Cysteine and penicillamine, the use of the latter as an adjuvant is acceptable.

following composition can be used:

cysteine. HCl 360 mg
penicillamine 341 mg
borax 400 mg
Water to prepare 10 ml

using membrane filtration.
Shelf life 10 days.