This study investigated the metabolites of ibudilast, phosphodiesterase 4 inhibitor. The information is important to examine illicit use in doping control. However the following modifications are required to improve this manuscript

Throughout the manuscript, abbreviations such as LC, MS, and PDE4 must be defined at their first appearance and used consistently thereafter.

As a technical term, 'protonated molecule' is correct, not 'protonated ion.

ESI and NCE must be defined.

Instrumentation

Table 1: The table provides insufficient information. The mobile phases are already described in the main text. The table should be deleted, and the flow rate and gradient program should be described in the main text.

Method

Specify which solution was used for pH adjustment

A container for a urine sample should be referred to as a 'tube,' not a 'beaker

G force should be expressed as g.

Brand names and company names for general tools and experimental instruments (e.g., vial, tube, shaker, centrifuge…) should be omitted unless they are used for a specific purpose

Analyte extraction from equine urine -Phase II

Is 'the sample' referring to the original urine sample or the supernatant urine sample mentioned in the previous section?

The cartridge was eluted…and dried… should be modified to the analytes were eluted…and the solvent was dried

Was an internal standard added?

The procedure is unclear; please rephrase it

Results

In Figure 1, metabolites are depicted with arrows indicating their biotransformation pathways—i.e., from Phase I to Phase II. The figure highlights which metabolites are in vivo or in vitro and whether they were detected in post-administration urine or blood samples.

Please specify the major metabolites for each method (i.e. in vivo, in vitro) and include their relative abundance

Explain how the position of methylation for M17 and M18 was determined. Other OH groups can also be O-methylated. If the position is uncertain, do not specify it.

In the third paragraph, the phrase "the methyl group from the isopropyl side chain" is inaccurate because ibudilast contains two isopropyl groups. Please specify which isopropyl