Treatment of malignant (metastatic) pheo is a big challenge.
Malignant pheo has a very variable clinical course; some patients live with it
for many years with high quality of life while some other patients can have a
rapid deterioration of their conditions. Malignant pheo is exceedingly rare,
making it hard for clinical trials to recruit patients. There have been no
approved therapies for malignant pheo so far.
Two recent advances may offer hope. They are both “targeted
internal radiotherapies”. The radiopharmaceuticals only go into certain cells
of the body, malignant pheo cells in particular (hence “targeted”). The
patients would receive intravenous infusions of the radiopharmaceuticals rather
than getting radiation from an external source (hence “internal”).
The first one is carrier-free MIBG radiotherapy. We don’t
have to go into the technical details of what is carrier-free but this new form
of MIBG radiotherapy supposedly deliver more radiation to the tumors. The
manufacturer released a press announcement this year, showing the effects of
carrier-free MIBG radiotherapy in patients with malignant pheo. Older forms of
MIBG radiotherapy have been used for years in clinical practice and in some
clinical trials. It is hard to compare the therapeutic effects of the old and
new MIBG radiotherapies. Generally the effects are comparable in my eyes. The
carrier-free MIBG radiotherapy is tested in a prospective and more controlled
manner, thus more convincing.
The second one is PRRT. Although the completed US clinical
trial of PRRT is only for carcinoid, PRRT has been used in Europe and Australia
for all kinds of neuroendocrine tumors, including malignant pheo. A recently
published paper showed that PRRT helps control hypertension and reduce the size
of malignant pheo. This study is a retrospective one so it is even harder to
compare the carrier-free MIBG radiotherapy and PRRT. My own cursory reading
convinces me that they have roughly similar efficacy.
The carrier-free MIBG radiotherapy is seeking US approval. PRRT
should be close to approval in the US but will very likely be approved for
carcinoid only. Assuming both will be approved eventually, I imagine it will be
interesting to how they are used for patients with malignant pheo. The use of
PRRT will be “off-label”. Besides efficacy, availability, comorbidities, side
effects, cost, and insurance coverage will also be factors that influence the
decision to use one over the other. More interestingly, can the two
radiotherapies be used in succession? The
best indication of the two radiotherapies should be assessed on a case-by-case
basis.
Dr. Pheo
