Sales of drugs used to treat bipolar disorder are set to tumble in seven major markets, declining from a value of $6.3 billion in 2011 to just over $4 billion by 2021, according to new forecasts.

Generic competition within the atypical antipsychotic drug class will cause "a precipitous decline" in sales of these products in the US, France, Germany, Italy, Spain, the UK and Japan over the period, adds the study, from Decision Resources.

Uptake of Dainippon Sumitomo Pharma/Sunovion/Takeda Pharmaceutical's Latuda (lurasidone) and the launch of Forest Laboratories/Gedeon Richter/Mitsubishi Tanabe Pharma's cariprazine (RGH-188) will help temper the decline in sales of atypical antipsychotics, but will not be sufficient to offset generic erosion within this drug class, it forecasts.

The market for drugs used to treat bipolar disorder is dominated by the US, which accounted for nearly 90% of major-market sales of such treatments last year. Decision Resources says it expects sales of antipsychotic drugs in the major markets within Europe plus Japan to remain relatively stable, overall, because the increase in patient share in these regions, including the share of branded emerging antipsychotics, will help to offset the effects of generic erosion.

Total class sales of atypical antipsychotics will decline to just over $3 billion by 2021 in the seven major markets, it forecasts.

The study points out that more effective remission of bipolar depression is the largest unmet need in bipolar disorder, with experts consistently stating few therapies currently achieve sufficient efficacy in this market segment and that, therefore, this presents the greatest commercial opportunity in bipolar disorder.

"We forecast that Latuda, which launched for the treatment of schizophrenia in the US in 2011, will gain US approval for bipolar depression in 2013 and will subsequently launch in 2014 in Europe and Japan," says Alana Simorellis, an analyst at Decision Resources. 

"Once approved, marketers will differentiate the drug based on its efficacy in bipolar depression and its relatively mild weight gain profile compared with currently-marketed atypical antipsychotics. We expect Latuda will make significant contributions to the bipolar depression market, with peak-year sales of $500 million to $1 billion," she forecasts.

The study findings also reveal that the treatment algorithm of bipolar disorder is largely unchanged – with lithium, mood stabilisers and atypical antipsychotics remaining the primary treatments. Although physicians are aware of the controversy over prescribing antidepressants to this patient population, they continue to do so because of the lack of therapies for bipolar depression. Sales of antidepressants - selective serotonin reuptake inhibitors (SSRIs), serotonin and norepinephrine reuptake inhibitors (SNRIs), dopaminergic/noradrenergic agents, serotonin antagonists/reuptake inhibitors (SARIs) and melatonin receptor agonists - accounted for only 8% of the market last year, reaching just $480 million in total sales, because generic versions of many of these agents are available, it says.