Background: Pazopanib is an approved treatment for renal cell carcinoma (RCC) and soft tissue sarcoma (STS). At the currently registered fixed dose of 800 mg QD, 20% of patients (pts) do not attain the efficacy threshold of C_min≥ 20.5 mg/L (Suttle et al, 2014), providing a strong rationale for therapeutic drug monitoring (i.e. individualizing the dose based on measured plasma drug concentrations). Previous studies provided cost-neutral alternatives to absolute dose increments to optimize exposure (i.e. splitting intake moments or concomitant intake with food (Groenland et al, 2020; Lubberman et al, 2019)). This study aimed to investigate the feasibility, tolerability and efficacy of TDM of pazopanib, using cost-neutral interventions. Methods: Patients starting treatment with pazopanib at the standard dose of 800 mg QD in modified fasting state were included in the prospective DPOG TDM study (www.trialregister.nl, NL6695). PK sampling occurred 4, 8 and 12 weeks after start of treatment, and every 12 weeks thereafter. Pazopanib concentrations were measured with LC-MS/MS and C_min was calculated. In case of C_min< 20.5 mg/L and acceptable toxicity, a dose intervention was recommended. As a first step, intake moments were split (i.e. 400 mg BID). Secondly, concomitant intake with food was recommended. Results: In total, 34 pts were included (19 STS, 15 RCC), of whom 158 PK samples were collected. Eleven pts (32%) were underdosed and had at least 1 PK sample below the target. In 24% of the pts a PK-guided intervention could be performed, which was successful in 6 pts (75%). Median C_min increased from 15 mg/L to 32 mg/L (p = 0.027). Eventually, 3 pts went back to 800 mg QD due to toxicity, after which C_min remained ≥ 20.5 mg/L in 2 pts. In pts with adequate exposure throughout the study, median C_min was 32 mg/L (range 23 – 65 mg/L). In 3 pts, a PK-guided intervention could not be performed, due to progression (n = 1) or logistical issues (n = 2). Twelve pts (35%) received a dose reduction due to toxicity (lowest dose was 200 mg QAD), exposure remained adequate at this reduced dose in all pts. For STS pts, median PFS was 19.8 months in pts with C_min< 20.5 mg/L who did need an intervention vs. 6.4 months in pts with all C_min≥ 20.5 mg/L (not significant). For RCC pts, this was 15.5 months vs. 7.4 months, respectively (not significant). Conclusions: This prospective study shows that PK-guided dose optimization of pazopanib using cost-neutral interventions is feasible in daily practice. A PK-guided intervention was performed in 24% of the patients, which was successful in 75% of these patients. Clinical trial information: NL6695.