The synthesis of rac-PZQ via the Pictet-Spengler route was developed by the Korean Shin Poong Pharmaceutical Company and obtains very low production costs of US¢7 per 600 mg tablet of the drug.

First step (amide bond formation to give 3)
Second step (attachment of acetal to give 4) - there's also a one-pot procedure to combine the first 2 steps
Third step (cyclization to praziquanamine to give 5)
Fourth step...
Combination of steps 1-3 to give the dimethoxy analog of PZQ
 
Key references:
[1]    Formation of pyrazinoisoquinoline ring system by the tandem amidoalkylation and N-acyliminium ion cyclization: An efficient synthesis of Praziquantel, J. H. Kim, Y. S. Lee, H. Park and C. S. Kim, Tetrahedron 1998, 54, 7395-4000.
 
[2]    Synthesis of Praziquantel via N-Acyliminium Ion Cyclization of Amido Acetals through Several Synthetic, J. H. Kim, Y. S  Lee and C. S. Kim, Heterocycles 1998, 48, 11, 2279-2285.
 
[3]   History of Praziquantel: http://www.stanford.edu/group/parasites/ParaSites2006/Praziquantel/history.html