wiki has your answers for success rate at different...

wiki has your answers for success rate at different phases: https://en.wikipedia.org/wiki/Phases_of_clinical_research

Summary of clinical trial phases
Phase	Primary goal	Dose	Patient monitor	Typical number of participants	Success rate[2]	Notes
Preclinical	Testing of drug in non-human subjects, to gather efficacy, toxicity and pharmacokinetic information	unrestricted	scientific researcher	not applicable (in vitro and in vivo only)
Phase 0	Pharmacokinetics; particularly, oral bioavailability and half-life of the drug	very small, subtherapeutic	clinical researcher	10 people		often skipped for phase I
Phase I	Testing of drug on healthy volunteers for safety; involves testing multiple doses (dose-ranging)	often subtherapeutic, but with ascending doses	clinical researcher	20–100 normal healthy volunteers (or for cancer drugs, cancer patients)	approximately 70%	determines whether drug is safe to check for efficacy
Phase II	Testing of drug on patients to assess efficacy and side effects	therapeutic dose	clinical researcher	100–300 patients with specific diseases	approximately 33%	determines whether drug can have any efficacy; at this point, the drug is not presumed to have any therapeutic effect whatsoever
Phase III	Testing of drug on patients to assess efficacy, effectiveness and safety	therapeutic dose	clinical researcher and personal physician	300–3,000 patients with specific diseases	25–30%	determines a drug's therapeutic effect; at this point, the drug is presumed to have some effect
Phase IV	Postmarketing surveillance – watching drug use in public	therapeutic dose	personal physician	anyone seeking treatment from their physician	N/A	watch drug's long-term effects