3060 Background: The determination of the optimal biological dose (OBD) defined as the lowest safe dose associated with biological efficacy, appears to be a promising endpoint for defining the recommended phase 2 trial dose (RP2D) of novel oncologic targeted therapies in early-phase clinical trials. However, the clinical relevance of OBD is still unknown. We conducted a review to assess if the OBDs of molecular targeted therapies defined during early phase trials were useful during subsequent drug development and for oncologic drug approvals. Methods: A systematic review was conducted to identify all the molecular targeted therapies approved by FDA in solid and hematological malignancies, and for which early phase trials defined OBDs. The publications of efficacy trials leading to the first FDA approvals were reviewed, as were the FDA approved doses and dosing schedules, which were compared to OBDs found in the early phase trials. Results: OBDs were reported in the early phase trial articles of 39.5% (32/81) FDA approved targeted therapies from 1999 to 2017 (19 small molecules and 13 monoclonal antibodies (mAbs)). The maximum tolerated doses (MTD) had not been reached for 59.4% (19/32) of these drugs. When both MTD and OBD had been defined, OBD were lower than MTD in 84.6% of cases, and equal for the others. The OBDs were chosen as the RP2Ds for 56.3% of the molecules. In that case, the final FDA approved doses were consistent with the OBDs for 83.3% of the drugs. These percentages did not differ in between small molecules and mAbs. OBDs mainly relied on indirect effects on the involved signaling pathway elements for small molecules (11/19, 57.9%), and on involved receptor occupancies for mAbs (6/13, 46.2%). In total, 23.5% of all FDA approved molecular targeted therapies were approved at their OBDs. Conclusions: Although still poorly investigated, OBD may represent a relevant complementary endpoint in in early phase trials of novel anti-cancer targeted therapies, as OBDs are selected as the final FDA approved doses in 83.3 % of cases when chosen as the RP2Ds, which is much higher than the previously reported 58.0 % when MTDs are chosen as the RP2Ds (Fontes-Jardim et al. JNCI 2015).
