Here is an brief and oversimplified example of a s

Here is an brief and oversimplified example of a stunningly successful phase one trial that did not show measurable tumor reduction: New molecule with good preclinical data trialed on people with various, advanced end-stage cancers. The drug is tolerated fairly well - minimal dropout rate and no significant negative effects seen as dose increases thru cohort maturations. DLT in 7th cohort. MTD established at 6th cohort dosing level. Disease progression slowed in cohorts 5 and 6, although no measurable tumor reduction occurs. Scant but measurable reduction in biomarkers at cohort 5 dose level exists. More significant biomarkers reduction for (listed specific cancers) occurs in cohort 6 at mtd. Drug proves, thru biomarker reduction, to target (listed specific cancers) successfully in humans and is well tolerated by patients at established treatment dose.

Now - a phase II trial is initiated - utilizing patients with cancers specific to known biomarker reductions seen in phI, starting at dose established in phI to garner measurable response. PhII trial will more closely measure efficacy at established treatment dose. PhII trial will provide further safety and tolerability data as well as establish optimal frequency and length of treatment. Desired result of tumor reduction will be seen in the therapeutically dosed phII trial.

So - no tumor reduction in PhI not necessarily a negative by any means. Often - no tumor reduction is simply result of lack of optimal treatment time at established therapeutic dose.