Gross profit for the six months ended April 30, 2016 decreased $22.0 million, or 8.5%, to $237.2 million, from $259.2 million for the six months ended April 30, 2015. The decrease in gross profit margin from 30.0% for the six months ended April 30, 2015 to 27.1% for the six months ended April 30, 2016 was primarily due to FDA remediation activities at our Ferentino, Italy site, the absence of BLS gross profit due to the spinoff of the BLS business on July 31, 2015, and increased investment in quality across the network, partially offset by growth in our DSS business and operational excellence savings. On a constant currency basis, gross profit would have decreased $26.1 million.
Selling, general and administrative expenses for the six months ended April 30, 2016 decreased $2.4 million, or 1.6%, to $148.5 million, from $150.9 million for the six months ended April 30, 2015. The spinoff of the BLS business on July 31, 2015 reduced selling, general and administrative expenses by $9.0 million, which was partially offset by additional selling, general and administrative expenses from the Irix and Agere acquisitions. Additionally, we incurred increased consulting costs due to several corporate initiatives. On a constant currency basis, selling, general and administrative expenses would have increased $1.3 million, or 0.9%.
Research and development expenses for the six months ended April 30, 2016 decreased $8.1 million to $1.3 million, from $9.4 million for the six months ended April 30, 2015. Research and development expenses relate to proprietary research and development efforts and consist of salaries and benefits, supplies, outside services, and other costs. The decrease is primarily due to the spinoff of our BLS business on July 31, 2015, which contributed to the majority of our historical research and development expenses.
Repositioning expenses for the six months ended April 30, 2016 decreased $15.7 million, to $2.5 million, from $18.2 million for the six months ended April 30, 2015. Repositioning expenses for the six months ended April 30, 2016 primarily related to severance payments resulting from a realignment of DPS operations, while repositioning expenses for the six months ended April 30, 2015 primarily related to the cancellation of the DSM transaction service agreements, which resulted in an expense of $16.3 million for the period.
Acquisition and integration costs for both the six months ended April 30, 2016 and the six months ended April 30, 2015 was $10.2 million. Acquisition and integration costs during the six months ended April 30, 2016 primarily related to due diligence on potential corporate development transactions while acquisition and integration costs for the six months ended April 30, 2015 related to integration of Irix and Agere, as well as continued integration of 2014 acquisition activity. The nature of these costs includes due diligence costs, consultants, system and customer conversions, and other integration-related costs. These costs are recognized as operating expenses as incurred.
Other operating income for the six months ended April 30, 2016 increased $3.4 million, to $3.7 million, from a $0.3 million loss for the six months ended April 30, 2015. Other income for the six months ended April 30, 2016 related to $4.9 million of income from renegotiating the Biologics earnout in the first quarter of fiscal 2016, offset by $1.2 million in expenses due to marking the Biologics earnout to its estimated fair value at the end of the quarter.
Operating income for the six months ended April 30, 2016 increased $7.6 million to $78.4 million from an operating loss of $70.8 million for the six months ended April 30, 2015 as a result of the factors discussed above.
Interest expense, net
Interest expense for the six months ended April 30, 2016 was $86.4 million compared to $56.6 million for the six months ended April 30, 2015. The increase in interest expense is primarily due to additional debt borrowings in March 2015 and the issuance of the Senior PIK Toggle Notes in May 2015, as further discussed in the “Liquidity and Capital Resources” section below.
Refinancing expense
Refinancing expense for the six months ended April 30, 2015 was $3.7 million, which related to refinancing our Credit Facility in March 2015 to finance the Irix Acquisition. The company did not incur any refinancing expenses for the six months ended April 30, 2016.