The invention provides a drug conjugate comprising: 1) a therapeutic agent that comprises a phosphate, phosphonate, carboxy, or phosphoramidate group, and 2) a fluorescent group linked to the phosphate, phosphonate, carboxy, or phosphoramidate group to form the corresponding ester and salts thereof. The conjugates are useful for therapy and as probes. The invention also provides therapeutic methods for treating diseases with conjugates of the invention as well as methods for determining optimal dosages of a conjugate or a therapeutic agent for a given patient using a conjugate of the invention
