So sorry to hear the news Buff.
As it's small cell, and by 'spread' I assume you mean it's spread to other sites than the lungs.
In considering the two trials, it's worth bearing in mind the potential advantages (highly limited from the information you've posted) against the disadvantages of side effects.
With Irinotecan there can be psychotropic effects - (anxiety and panic attacks, headaches and chest pain to name a few).
With Paclitaxel they're less challenging (nausea, aching joints etc.)
Neither of them claim any 'miracle cure' status for late-stage cancers I'm afraid.
My honest opinion is that your main priority should be to work with his Oncologist to make sure he has a robust Pain Management Regime in place. In my experience it's not unusual for this to lack detail and attention, but it's really vital.
With Secondary Metastases, and the primary diagnosis, the prognosis isn't positive in terms of length of life. If that is the case, it's very difficult, but the kindest thing is to put energy into the quality, rather than the length of life.
Ask your Oncologist questions about pain management. Opiates, usually Methadone Sulphate Tablets (MSTs) are often the favored option and are highly effective, but do mean the patient can spend a great deal of time 'zonked'. I know that where patients and their family have acknowledged the finality of the situation, they often seek alternative pain management options that allow them to be more aware during the final days with their loved ones. This sometimes includes complementary medicine as well as traditional medicine. The options may be limited depending on your Father's specific condition, but all Oncologists I work with are happy to work with the patient and family who take an interest in the various options.
Another factor to talk through with your Father and the family, if his condition is terminal, is his preference about where he would like to die. A huge number of people I've worked have chosen to pass away at home with their loved ones rather than in an impersonal hospital setting. This isn't always an available option depending on the intensity of the care he needs, but if you're able to face that conversation, it can make a big difference to how you all cope with the situation, now, and in the long term.
A "half-way house" option is a Hospice, but I plead ignorance about their availability in the US.
I'm so sorry that the news from your Oncologist isn't good, and I hope you don't mind me being as frank as I have, but the advice is based on working with many people facing the circumstances you're in. I do hope things go as well as they can for you and your family.
Now I need to redeem my reputation as a cold and insensitive ******* so I'll find some other threads to flame people.
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"I started out with nothin' and I still got most of it left" - Seasick Steve