The efficacy of VORAXAZE was evaluated in a subset of 22 patients enrolled in Study 1 (NCT00001298), a single-arm, open-label study in patients who had markedly delayed methotrexate clearance (defined as more than 2 standard deviations greater than the mean excretion curve for methotrexate) due to impaired renal function. All patients received VORAXAZE 50 Units/kg as an intravenous injection over 5 minutes; those patients with pre-VORAXAZE methotrexate concentration >100 μmol/L were to receive a second dose of VORAXAZE 48 hours after the first dose. The protocol specified that patients continue receiving intravenous hydration, urinary alkalinization and leucovorin and that leucovorin administration be adjusted to ensure that it was not administered within 2 hours before or after VORAXAZE. These 22 patients had a pre-VORAXAZE methotrexate concentration >1 μmol/L and both pre- and post-treatment plasma samples available for determination of methotrexate concentration by a chromatographic method. The main outcome measure was the proportion of patients who achieved a rapid and sustained clinically important reduction (RSCIR) in plasma methotrexate concentration, defined as an attainment of plasma methotrexate concentration ≤1 μmol/L at 15 minutes that was sustained for up to 8 days following the initial injection.

The median age was 15.5 years (5 to 84 years); 59% were male; and the most common underlying cancers were osteogenic sarcoma (50%) and leukemia or lymphoma (45%).

Ten of the 22 patients achieved a RSCIR [45% (95% CI: 27%, 65%)]. Of the 12 patients who failed to achieve RSCIR, 5 patients (23%) attained a transient plasma methotrexate concentration ≤1 μmol/L. In these 5 patients, the median increase of plasma methotrexate concentration from their nadir was 1.4 μmol/L (0.3 to 2.5 μmol/L).

Table 2 summarizes the results of RSCIR and exploratory analyses following the first dose of VORAXAZE. An exploratory analysis in subgroups determined by pre-VORAXAZE methotrexate concentration suggests that the likelihood of attaining a RSCIR following the first VORAXAZE dose correlates with the pre-VORAXAZE methotrexate concentration. An additional exploratory analysis showed that all 9 patients with pre-VORAXAZE methotrexate concentration >50 μmol/L achieved >95% reduction in methotrexate concentration for up to 8 days following the first VORAXAZE dose although none of them achieved a RSCIR.

Lack of Efficacy with a Second Dose of VORAXAZE

Six of the 7 patients with pre-first dose VORAXAZE methotrexate concentration >100 μmol/L received a second VORAXAZE dose of 50 Units/kg administered 48 hours after the first dose. Among them, none of the 4 patients with pre-second dose VORAXAZE methotrexate concentration >1 μmol/L achieved a RSCIR. The remaining 2 patients achieved a RSCIR, but their pre-second dose VORAXAZE methotrexate concentration were already ≤1 μmol/L.

Deaths Attributable to Methotrexate Toxicity

There are no controlled trials comparing VORAXAZE and supportive care to supportive care alone in patients with toxic plasma methotrexate concentration due to impaired renal function; therefore, there are no data regarding the effect of VORAXAZE on survival or toxic deaths due to methotrexate. VORAXAZE did not prevent fatal methotrexate toxicity in 3% of patients in the safety population.