TL/DR: There are no quality studies supporting the Pentoxyverine. The class of drugs that Pentoxyverine belongs to does not have strong support either.
Our good friends at the Cochrane Collaboration, who do high-quality meta-analyses, looked at over-the-counter cough medicines:
(Ambulatory settings roughly means medical care not in a hospital ward.)
They looked at twenty-five trials involving 3492 people (separating adults and children), and they looked at expectorants, antihistamines, antihistamine decongestants, antitussives, bronchodilators and guaifenesin.
The evidence for effectiveness of oral over-the-counter cough medicines is weak
Acute cough is a common and troublesome symptom in people who suffer from acute upper respiratory tract infection (URTI). Many people self-prescribe over-the-counter (OTC) cough preparations and health practitioners often recommend their use for the initial treatment of cough. The results of this review suggest that there is no good evidence for or against the effectiveness of OTC medications in acute cough. The results of this review have to be interpreted with caution because the number of studies in each category of cough preparations was small. Many studies were of low quality and very different from each other, making evaluation of overall efficacy difficult.
Now, there were no trials included that covered Pentoxyverine. However, Pentoxyverine is an antitussive, and there were a total of 8 trials that covered antitussives. So the class of drugs was tested, and did not provide clear evidence for or against, but the individual drug wasn't covered.
Further, looking at their selection criteria, any high-quality studies of Pentoxyverine should have been found and included. We can conclude that a careful search by these researchers for quality trials of Pentoxyverine found none.